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In grammar, a modifier is an optional element in phrase structure or clause structure. A modifier is so called because it is said to modify (change the meaning of) another element in the structure, on which it is dependent. Typically the modifier can be removed without affecting the grammar of the sentence. For example, in the English sentence This is a red ball, the adjective red is a modifier, modifying the noun ball. Removal of the modifier would leave This is a ball, which is grammatically correct and equivalent in structure to the original sentence.
Other terms used with a similar meaning are qualifier (the word qualify may be used in the same way as modify in this context), attribute, and adjunct. These concepts are often distinguished from complements and arguments, which may also be considered dependent on another element, but are considered an indispensable part of the structure. For example, in His face became red, the word red might be called a complement or argument of became, rather than a modifier or adjunct, since it cannot be omitted from the sentence.
Premodifiers and postmodifiers 
Modifiers may come either before or after the modified element (the head), depending on the type of modifier and the rules of syntax for the language in question. A modifier placed before the head is called a premodifier; one placed after the head is called a postmodifier.
For example, in land mines, the word land is a premodifier of mines, whereas in the phrase mines in wartime, the phrase in wartime is a postmodifier of mines. A head may have a number of modifiers, and these may include both premodifiers and postmodifiers. For example:
- that nice tall man from Canada that you met
In this noun phrase, man is the head, nice and tall are premodifiers, and from Canada and that you met are postmodifiers.
Notice that in English, simple adjectives are usually used as premodifiers, with occasional exceptions such as galore (which always appears after the noun) and the phrases time immemorial and court martial (the latter comes from French, where most adjectives are postmodifiers). Sometimes placement of the adjective after the noun entails a change of meaning: compare a responsible person and the person responsible, or the proper town (the appropriate town) and the town proper (the area of the town as properly defined).
It is sometimes possible for a modifier to be separated from its head by other words, as in The man came who you bumped into in the street yesterday, where the relative clause who...yesterday is separated from the word it modifies (man) by the word came. This type of situation is especially likely in languages with free word order.
Types of modifiers 
The two principal types of modifiers are adjectives (and adjectival phrases and adjectival clauses), which modify nouns; and adverbs (and adverbial phrases and adverbial clauses), which modify other parts of speech, particularly verbs, adjectives and other adverbs, as well as whole phrases or clauses. (Not all adjectives and adverbs are necessarily modifiers, however; an adjective will normally be considered a modifier when used attributively, but not when used predicatively – compare the examples with the adjective red at the start of this article.)
Another type of modifier in some languages, including English, is the noun adjunct, which is a noun modifying another noun (or occasionally another part of speech). An example is land in the phrase land mines given above.
Examples of the above types of modifiers, in English, are given below.
- It was [a nice house]. (adjective modifying a noun, in a noun phrase)
- [The swiftly flowing waters] carried it away. (adjectival phrase, in this case a participial phrase, modifying a noun in a noun phrase)
- She's [the woman with the hat]. (adjectival phrase, in this case a prepositional phrase, modifying a noun in a noun phrase)
- I saw [the man whom we met yesterday]. (adjectival clause, in this case a relative clause, modifying a noun in a noun phrase)
- His desk was in [the faculty office]. (noun adjunct modiying a noun in a noun phrase)
- [Put it gently in the drawer]. (adverb in verb phrase)
- He was [very gentle]. (adverb in adjective phrase)
- She set it down [very gently]. (adverb in adverb phrase)
- [Even more] people were there. (adverb modifying a determiner)
- It ran [right up the tree]. (adverb modifying a prepositional phrase)
- [Only the dog] was saved. (adverb modifying a noun phrase)
In some cases, noun phrases or quantifiers can act as modifiers:
- [A few more] workers are needed. (quantifier modifying a determiner)
- She's [two inches taller than I]. (noun phrase modifying an adjective)
Ambiguous and dangling modifiers 
Sometimes it is not clear which element of the sentence a modifier is intended to modify. In many cases this is not important, but in some cases it can lead to genuine ambiguity. For example:
- He painted her sitting on the step.
Here the participial phrase sitting on the step may be intended to modify her (meaning that the painting's subject was sitting on the step), or it may be intended to modify the verb phrase painted her or the whole clause he painted her (or just he), meaning in effect that it was the painter who was sitting on the step.
Sometimes the element which the modifier is intended to modify does not in fact appear in the sentence, or is not in an appropriate position to be associated with that modifier. This is often considered a grammatical or stylistic error. For example:
- Walking along the road, a vulture loomed overhead.
Here whoever was "walking along the road" is not mentioned in the sentence, so the modifier (walking along the road) has nothing to modify, except a vulture, which is clearly not the intention. Such a case is called a "dangling modifier", or more specifically, in the common case where (as here) the modifier is a participial phrase, a "dangling participle".