In grammar

Grammar

In linguistics, grammar is the set of structural rules that govern the composition of clauses, phrases, and words in any given natural language. The term refers also to the study of such rules, and this field includes morphology, syntax, and phonology, often complemented by phonetics, semantics,...

, a modifier is an optional element in phrase

Phrase

In everyday speech, a phrase may refer to any group of words. In linguistics, a phrase is a group of words which form a constituent and so function as a single unit in the syntax of a sentence. A phrase is lower on the grammatical hierarchy than a clause....

 structure or clause

Clause

In grammar, a clause is the smallest grammatical unit that can express a complete proposition. In some languages it may be a pair or group of words that consists of a subject and a predicate, although in other languages in certain clauses the subject may not appear explicitly as a noun phrase,...

 structure; the removal of the modifier typically doesn't affect the grammaticality of the sentence.

In English, adverb

Adverb

An adverb is a part of speech that modifies verbs or any part of speech other than a noun . Adverbs can modify verbs, adjectives , clauses, sentences, and other adverbs....

s and adjective

Adjective

In grammar, an adjective is a 'describing' word; the main syntactic role of which is to qualify a noun or noun phrase, giving more information about the object signified....

s prototypically function as modifiers, but they also have other functions. Moreover, other can function as modifiers as the following examples show :

• [Put it gently in the drawer]. (adverb in verb
  Verb
  A verb, from the Latin verbum meaning word, is a word that in syntax conveys an action , or a state of being . In the usual description of English, the basic form, with or without the particle to, is the infinitive...
  
   phrase)
• She set it down [very gently]. (adverb in adverb phrase)
• He was [very gentle]. (adverb in adjective phrase)
• [Even more] people were there. (adverb in determiner
  Determiner (class)
  A determiner is a noun-modifier that expresses the reference of a noun or noun-phrase in the context, rather than attributes expressed by adjectives...
  
   phrase)
• It ran [right up the tree]. (adverb in prepositional phrase)
• It was [a nice house]. (adjective in noun phrase)
• His desk was in [the faculty office]. (noun in noun phrase)
• [The swiftly flowing waters] carried it away. (verb phrase in noun phrase)
• I saw [the man whom we met yesterday]. (clause in noun phrase)
• She's [the woman with the hat]. (preposition phrase in noun phrase)
• It's not [that important]. (determiner in adjective phrase)
• [A few more] workers are needed. (determiner in determiner phrase)
• We've already [gone twelve miles]. (noun phrase in verb phrase)
• She's [two inches taller than I]. (noun phrase in verb adjective phrase)

A premodifier is a modifier placed before the head (the modified component). A postmodifier is a modifier placed after the head, for example:

• land mines (pre-modifier)
• mines in wartime (post-modifier)
• time immemorial (post-modifier)

A few adjectives, borrowed from French, may be postmodifiers, generally with a change in meaning from their premodifier use. An example is proper:

They live in a proper town (in a real town)

They live in the proper town (in the town that's right for them)

They live in the town proper (in the town itself)