Modifiers

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.

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:

In this noun phrase, man is the head, nice and tall are premodifiers, and from Canada and whom you met are postmodifiers.

Notice that in English, simple adjectives are usually used as premodifiers, with occasional exceptions such as (which always appears after the noun) and the phrases and (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).

...
Wikipedia