Grammatical gender in Spanish
Grammatical gender in Spanish affects several types of words (and their mutual agreement) which have inflection in the Spanish language according to grammatical gender: nouns, adjectives, determiners, and pronouns. All Spanish nouns have lexical gender, either masculine or feminine, and most nouns referring to male humans or animals are grammatically masculine, while most referring to females are feminine. In terms of markedness, the masculine is unmarked and the feminine is marked in Spanish.
Most gender features are common to all of the Romance languages, though in comparison Spanish kinship terminology derives more female terms from male terms: for example, uncle/aunt is tío/tía in Spanish but oncle/tante in French; brother/sister is hermano/hermana in Spanish but fratello/sorella in Italian. Spanish personal pronouns also uniquely distinguish feminine forms of the first and second person plural (nosotras, vosotras).
The most common genders are called masculine and feminine, while some Spanish pronouns are considered to have neutral gender. A few nouns are said to be of "ambiguous" gender, meaning that they are sometimes treated as masculine and sometimes as feminine. Additionally, the terms "common gender" and "epicene gender" are used to classify ways in which grammatical gender interacts (or not) with "natural gender" (the gender identity of a person, or the sex of an animal). Adjectives ending in -o are almost always masculine,  and they always have a feminine counterpart ending in -a; a number of adjectives ending in -a, such as those ending en -ista, are both masculine and feminine.
The masculine (masculino): As a general rule, nouns ending in -o (libro 'book', zapato 'shoe') and nouns which refer to males (profesor, padre 'father') are masculine. Exceptionally, mano ('hand') is feminine. Also some colloquial shortened forms of feminine nouns end with -o: la foto(grafía) ('photograph'), la disco(teca) ('discothèque'), la moto(cicleta) ('motorcycle'), la radio(difusión) ('radio [broadcasting]').
The feminine (femenino): As a general rule, nouns ending in -a (casa 'house', boca 'mouth') and nouns which refer to females (madre 'mother', mujer 'woman, wife') are feminine. Similarly, the endings -ción, -sión, -dad, -tad, and -umbre indicate feminine gender. Exceptionally, día ('day'), mapa ('map') and sofá ('sofa') are masculine. Likewise, nouns of Greek origin ending in -ma (drama, problema) or "-ta" ("planeta", "profeta") are masculine. (These "Greek" nouns can often be identified by their derived adjectives ending in -tico.)
"Common gender" (común) is the term applied to those nouns, referring to persons, that keep the same form regardless of the sex of the person, but which change their grammatical gender. For example, el violinista ('the male violinist'), la violinista ('the female violinist'), el mártir ('the male martyr'), la mártir ('the female martyr'), el testigo ('the male witness'), la testigo ('the female witness'), el espía ('the male spy'), la espía ('the female spy'), etc. To this gender belong present participles derived from active verbs and used as nouns, such as el estudiante ('the male student'), la estudiante ('the female student'), el atacante ('the male attacker'), la atacante ('the female attacker'), el presidente ('the male president'), la presidente ('the female president'—although la presidenta is also often used), etc.
"Epicene gender" (epiceno) is the term applied to those nouns that have only one grammatical gender, masculine or feminine, but can refer to a living creature of either sex. Most animal names are of this type. E.g.: el ratón ('mouse'), la rata ('rat'), la rana ('frog'), la comadreja ('weasel'), la liebre ('hare'), la hormiga ('ant'), el búho ('owl'), el escarabajo ('beetle'), el buitre ('vulture'), el delfín ('dolphin'), el cóndor ('condor'), la paloma ('dove'), la llama ('llama'). To specify sex, a modifying word is added, with no change of gender: el delfín macho ('the male dolphin'), el delfín hembra ('the female dolphin'), la comadreja macho, la comadreja hembra (male and female weasels respectively).
Ambiguous nouns (ambiguo) whose grammatical gender varies in usage are said to be of "ambiguous" gender. Often the change of gender brings about a change of connotation. E.g.: el mar ('the sea'), la mar ('the sea', poetic or among sailors), el calor ('heat'), la calor (archaic), el azúcar, la azúcar ('sugar').
The pronoun ello ('it, the aforementioned concept'), the demonstrative pronouns esto ('this [idea or unnamed thing]'), eso ('that'), and aquello ('that'), and some uses of the clitic object pronoun lo, are traditionally called "neuter" ("neutro") because they do not have a gendered noun as their antecedent, but rather refer to a whole idea, a clause, or an object that has not been named in the discourse.
Similarly, the article lo (not to be confused with the object pronoun lo) is not used with nouns, but rather with adjectives to create abstract nominal phrases: lo bueno, the good part (of it); lo importante, what is important (about it); it's also combined with the relative pronouns que and cual to form relative clauses, such as lo que dices, lo cual es cierto, and can also be followed by de, e.g. lo de Juan está aquí, lo de que estoy enfermo no es cierto.
In the recent history of the Spanish language, there is a unidirectional tendency for words with unusual gender to be regularized by analogy to other words of their class. For example, the word idioma ("language") is masculine in standard Spanish, due to deriving from the Greek where words ending in -ma are typically masculine. However it has become feminine in some dialects due to the fact that words ending in -a are typically feminine in Spanish.
Some feminist movements and ideologies have criticized certain grammatical rules in Spanish which use grammatically-masculine forms rather than grammatically-feminine forms. These include the grammatical custom (inherited from Latin) of using a grammatically-masculine plural for a group containing at least one biological male; the use of the masculine definite article for infinitives (e.g. el amar, not la amar); and the permissibility of using Spanish male pronouns for female referents but not vice versa (e.g. el que includes women, la que does not include men). There also exist solely-masculine apocope forms (e.g. al ("to him", from a + el), del ("of him", from de + el), algún (from alguno) and buen (from bueno)) simply due to inherited tendencies in phonology and morphology. Some early proposals for gender neutrality in Spanish have included extending the use of the gender-neutral -es ending for plural nouns, so that mis hijos ("my children") becomes mis hijes if they are of more than one gender, or non-binary).
Sometimes even feminist proposals can be constructed as sexist. Monoparental is a neologism formed from mono- ("one") and the Latin parentalis (Spanish pariente means "relative", English parent is progenitor or progenitora) to mean "single-parent". It has been occasionally analyzed as too similar to padre ("father"), causing the coining of monomarental to mean "single-mother". 
- Gender of nouns in Spanish - Gender rules
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- "monomarental". www.fundeu.es (in Spanish). Fundéu. Retrieved 18 May 2020.