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The Spanish language has nouns that express concrete objects, groups and classes of objects, qualities, feelings and other abstractions. All nouns have a conventional grammatical gender. Countable nouns inflect for number (singular and plural). However, the division between uncountable and countable nouns is more ambiguous than in English.
All Spanish nouns have one of two grammatical genders: masculine and feminine (mostly conventional, that is, arbitrarily assigned). Most adjectives and pronouns, and all articles and participles, indicate the gender of the noun they reference or modify.
In a sentence like "Large tables are nicer", the Spanish equivalent, Las mesas grandes son más bonitas, must use words according to the gender of the noun. The noun, mesa ("table"), is feminine in Spanish. Therefore, the article must be feminine too, and so la instead of el, is required. However, mesas is plural here, so we need las rather than la. The two adjectives, whether next to the noun or after the verb, have to "agree" with the noun as well. Grande is a word which is invariable for gender, so it just takes a plural marker: grandes. Bonito is a word that can agree for both gender and number, so we say bonitas to go with mesas. A student of Spanish must keep in mind all these features when making sentences.
Noun gender 
In general, most nouns that end in -a, -ción / -sión and -ad are feminine; the rest of the nouns, which usually end in -o or a consonant, are masculine.
Nouns can be grouped in the following categories:
- Applied to persons and most domesticated animals:
- Declinable nouns. The feminine form adds a or replaces the final vowel by a, e.g. el profesor/la profesora, el presidente/la presidenta, el perro/la perra. Often, nouns that refer to positions that are traditionally held by men are declinable.
- Invariant nouns (in Spanish, sustantivos de género común). The feminine form and the masculine form are identical: el artista/la artista, el testigo/la testigo, el estudiante/la estudiante.
- Nouns with a unique grammatical gender. The noun has a fixed gender, regardless of the sex of the person it describes: el personaje, la visita.
- Applied to wild and some domesticated animals:
- Nouns where the two sexes of animals have different words to describe them: el toro/la vaca, el caballo/la yegua.
- Epicene nouns. The gender of the noun is fixed and sex is indicated by macho (male) or hembra (female). Examples: la jirafa macho, la jirafa hembra, el rinoceronte macho, el rinoceronte hembra.
- Applied to things:
- Masculine, e.g. el pan.
- Feminine, e.g. la leche.
- Vacillant nouns (called sustantivos ambiguos in Spanish) accept either gender, e.g. linde ('boundary') and testuz ('animal's forehead'). Internet causes speakers to hesitate between making it masculine like other loanwords from English, or making it feminine to agree with red, 'net'. Meanwhile, azúcar ('sugar') can be masculine with el, feminine with la, or (oddly) feminine with el (perhaps as a carry-over from Old Spanish, in which the singular definite article was invariably el before nouns beginning with a-, regardless of gender and regardless of stress). Spanish is predominantly a masculine-based language. As such, the determiner seems to go in the masculine in standard use: el, este, ese, tanto, especially when referring to cases where gender is not specified. Any adjectives agreeing with it are usually masculine in Spain and feminine in Latin America: el azúcar moreno o blanco / el azúcar negra o rubia. Mar ("sea") is normally masculine, but in poetry and sailors' speech it is feminine. Arte is masculine in the singular and feminine in the plural, though it can be feminine in the singular when it means "art-form" and masculine in the plural in the expression los artes de pesca, "fishing gear".
- There is a pattern with words with an initial stressed /a/ sound, such as agua ("water"), that makes them seem ambiguous in gender, but they are not. Such words take the masculine article, both definite (el) and indefinite (un), in the singular form; they also take the singular modifiers algún (instead of alguna) and ningún (instead of ninguna) when those modifiers precede the nouns. Similar words include el alma / un alma ("soul"), el ala / un ala ("wing"), el águila / un águila ("eagle"), and el hacha / un hacha ("ax"). Still they are feminine and, as such, they take feminine modifiers (except those cases previously mentioned) in both singular and plural forms, and they take feminine articles in the plural form as in las aguas frías.
- Sometimes, two homonyms will differ in gender, e.g. el capital ("funds") and la capital ("capital city"); el cura ("the priest") and la cura ("the cure").
Determining gender from endings 
Nouns ending in -o are masculine, with the only notable exception of the word mano ("hand"); -a is typically feminine, with notable exceptions; other vowels and consonants are more often than not masculine, but many are feminine, particularly those referring to women (la madre) or ending in -ción/sión, -dad/tad, -ez (la nación, la televisión, la soledad, la libertad, la vejez).
A small set of words of Greek origin and ending in -ma, "-pa", or "-ta" are masculine: problema ("problem"), lema ("lemma, motto"), tema ("theme, topic"), sistema ("system"), telegrama ("telegram"), mapa ("map"), poeta ("poet"), planeta ("planet"), etc.
Words ending in -ista referring to a person can generally be either gender: el artista, la artista, "the artist, the female artist". The same is true of words ending in -ante or -ente, though sometimes separate female forms ending in -a are used.
Words taken from foreign languages may:
- take the gender they have in that language, with neuter taken to be the same as masculine (so English nouns are made masculine)
- take the gender it seems to be (e.g. la Coca-Cola because it ends in -a)
- take the gender of the closest-related Spanish word (e.g. la Guinness because of la cerveza)
Gender of proper nouns (names) 
Names of people 
People's names agree with the sex of the person, even if they appear to be the opposite:
- Chema es guapo
- Amparo es guapa
Names of settlements 
Usage for places varies. You can choose between making them:
- Feminine if they end in -a, otherwise masculine:
- la Barcelona de Gaudí
- el Londres de Dickens
- Agree with the underlying noun el pueblo or la ciudad
- Nueva York (city)
- la antigua Cartago (city)
- Fraga es pequeño (village/small town)
- Always masculine: (this usage may seem wrong to some speakers)
- Barcelona no es pequeño
- Londres no es pequeño
With examples like New York, the Nueva is a fixed part of the name and so cannot be made masculine, but New Mexico is translated as Nuevo México and considered masculine, since México is a masculine noun.
Rivers are masculine because of the underlying masculine noun río. The ancient Roman belief that rivers (amnes) were male gods may also influence this. Locally, a few rivers may be feminine, but the masculine is always safe and correct.
- el [río de la] Plata = "The River Plate" (literally "the River of Silver")
- el [río] Támesis = "The River Thames"
- el [río] Tajo = "The River Tagus"
- el [río] Colorado = "The Colorado River" (literally "the Red River")
- el [río] Cinca / la Cinca = "The River Cinca" (in the Aragonese Pyrenees)
Vestiges of a neutral gender 
While Spanish is generally regarded to have two genders, its ancestor, Latin, had three. The transition from three genders to two is mostly complete; however, vestiges of a neuter gender can still be seen. This was noted by Andrés Bello in his work on the grammar of Latin American Spanish. 
Most notably, this is seen in pronouns like esto, eso, aquello, and ello, which are the neuter forms of este, ese, aquel, and él, respectively. These words correspond with English "this", "that", "that" (more common than aquello), and "him". Additionally the word lo, while usually masculine, can be considered neuter in some circumstances. It can also be used in the place of el to be a neutral form of the article "the", as in lo mismo, "the same". Bello also notes that words such as nada, poco, algo, and mucho can be used as neuters in some contexts.
Neuter forms such as esto were preserved because unlike most nouns in Latin, the difference between masculine and neuter for these pronouns did not depend on a final consonant. For example, most second declension Latin neuter singulars in the nominative case ended in -um, the non-neuter counterpart often ending in -us. When the final consonants in these endings are dropped, the result is -u for both; this became -o in Spanish. However, a word like Latin iste had the neuter istud; the former became este and the latter became esto in Spanish.
Another sign that Spanish once had a grammatical neuter exists in words that derive from neuter plurals. In Latin, a neuter plural ended in -a, and so these words today in Spanish get interpreted as singular feminine, and take singular verb forms; however, they do express some notion of a plural. For example, la física corresponds to English "physics", a plural.
There are two grammatical numbers: singular and plural. The singular form is the one found in dictionaries (base form). The plural is indicated in most words by adding -s (if the base form ends in a vowel) or -es otherwise. Note that final y in words like rey, though phonetically a vowel, counts as a consonant (rey → reyes). The addition of -es to certain nouns produces changes in the placement of stress, thereby affecting the presence of accent marks (canción → canciones), and may cause a spelling change in a stem ending in -z (lápiz → lápices).
The masculine gender is used for plural forms of mixed sexes (it is inclusive): los niños, grammatically masculine, may mean "the children" or "the boys". The feminine gender is exclusive in the plural: las niñas = "the little girls". When male sex needs to be shown exclusively in the plural, phrases such as los niños varones are used. Feminists (and their satirists) try to reverse the pattern with phrases such as las personas humanas jóvenes varones = "the young male human people".
Some words are formally always grammatically plural: pantalones "trousers", tijeras "scissors". In many dialects, however, these words are taken to be semantic plurals, and their singular forms are used instead: pantalón, tijera.
In expressions with an indefinite determiner, singular forms are used (unlike English, where "some" and "any" tend to modify plural nouns).
- Si hay algún árbol, lo derribaremos = "If there is any tree, we will tear it down"
- Por cualquier medio = "By any means"
Forms of ninguno ("no") always take singular noun phrases, even where plurality might be intended:
- Ningún obstáculo se interpone = "No obstacle is in our way", "There are no obstacles in our way"
- No vi a ninguna mujer = "I saw no women", "I did not see any women"
The determiner cualquiera has a plural form (cualesquiera), but it is never used outside formal or technical contexts.
Diminutives, augmentatives and suffixes 
A very productive set of suffixes can be added to existing nouns and adjectives to form new Spanish nouns. This usually just slightly modifies the meaning, but sometimes it creates something new entirely.
The most common subset of such suffixes are the diminutives, which convey the idea of smallness, delicateness, etc. (also for endearing terms). The most common diminutive in Spanish is -it-. It is added to the root of the noun, and in actual usage, it takes the proper agreement for gender and number.
- planta → plantita / plantota ("plant" → "little plant" / "big plant")
- vaso → vasito / vasote ("glass" → "little glass" / "big glass")
- niño → niñito / niñote ("small boy" → "little tiny boy" / "Big (little tiny) boy")
In other cases, this ending can be pejorative or belittling.
- señor → señorito ("Sir/Mister" → "little sir/mister" (mockingly) compare (señora → señorita ("Madame/Mrs." → "Miss/Ms."))
When the word does not end in a vowel, -it- becomes -cit- for diminutives if the word ends in something other than an unstressed "-o" or "-a". Agreement marks are added to it according to the gender and number:
- botón → botoncito / botonote
- Carmen → Carmencita
- mamá → mamita, mamacita
- madre → madrecita
This is slightly modified when the base word ends in z. Because z and soft c are the same sound in Spanish, an epenthetic e is inserted (notice the orthographic change): pez → pececito / pecezote. There is nothing fixed when the base ends in other consonants: azúcar → azuquítar or azuquita / azucota.
When words end in -s or -te, there are varied approaches.
Idiomatic diminutives 
The choice of diminutive is often a mark of regional dialects and influence of coexistent Romance languages. Educated speakers who would use -ito / -ita or no diminutive at all in more formal speech may use local forms when they want a friendlier or more colourful way of expressing themselves, sometimes borrowing another region's diminutive.
So, instead of the standard -ito, you could find:
- -illo / -illa especially in Andalusia (’quillo for chico is a typical Cádiz interjection).
- -ico / -ica in Aragon, Navarra, Murcia, eastern Andalusia, parts of the Valencian Community, La Mancha ...
- a variant of this diminutive is used in many latinamerican countries, but only for nouns ending in -to, -ta or -te, while in other nouns -ito / -ita is used.
- -ín / -ina or -ino / -ina in the Spanish spoken in Asturias, as in Asturian or Bable.
- ín / -ina in Spanish spoken in Extremadura or León, as in Leonese.
- -iño / -iña in the Spanish spoken in Galicia, as in Galician.
- -uco / -uca in Cantabria.
- -eto / -eta in Aragon.
- -ete / -eta, possibly from Catalan, in much of eastern Spain.
- -uelo / -uela.
In fossilised forms, these can be found in standard words, such as puerta → portillo, burro → borrico, Venecia → Venezuela, paño → pañuelo, calle → calleja → callejuela etc.
Sometimes different suffixes are used for variety when more than one is used at once:
- chico → chiquito → chiquitillo etc.
There is a well-known tango called De chiquilin te miraba de afuera...
Other suffixes 
As well as being an Andalusian (especially Seville) alternative to -ito, the suffix -illo is also a special diminutive with a nuance of "a funny sort of...". It is also used to create new nouns:
- palo "stick" → palillo "toothpick"
- bolso "handbag" → bolsillo "pocket"
- guerra "war" → guerrilla "hit-and-run warfare"
An example of the same phenomenon, but using an augmentative, is -ón:
- soltero "bachelor" → solterón "confirmed bachelor"
- soltera "single woman" → solterona "spinster"
- puerta "door" → portón "gate" / "large door"
Another suffix that can either denote a blow with or be an augmentative is -azo:
- puerta ("door") → portazo ("slam of a door")
- mano ("hand") → manotazo ("a hit with the hand")
- cacerola ("saucepan") → cacerolazo (both "a blow with a saucepan" or "a big saucepan", also a form of protest)
- Bogotá (Bogotá, capital of Colombia) → Bogotazo (the "Bogotazo", the riots on April 9, 1948)
- Caracas (Caracas, capital of Venezuela) → Caracazo (the "Caracazo", the violent protests of 27 February 1989)
- derecha ("right hand") → derechazo (either a "right-hander" when slapping someone, or a "right-handed pass with the cape" in bullfighting)
- flecha ("arrow") → flechazo ("arrow shot" / "arrow wound", or figuratively "love at first sight")
|For a list of words relating to Spanish nouns, see the Spanish nouns category of words in Wiktionary, the free dictionary.|