Agreement [can occur] between nouns and their modifiers, in some [languages]. This is common in languages such as French, where articles, determiners and adjectives (both attributive and predicative) agree in number with the nouns they qualify:

 • le grand homme ("the great man") vs. les grands hommes ("the great men")

In English this is not such a common feature, although there are certain determiners that occur specifically with singular or plural nouns only:

 • One big car vs. Two big cars

 • Much great work vs. Many great works

[Where appropriate, an] adjective must always "agree" with its noun or pronoun by matching its number. Singular nouns take singular adjectives, while plural nouns take plural adjectives. For example, it would make no sense to say "two book," because "book" is singular while "two" is plural. We instead say "two books." Similarly, the phrase "a blue trees" lacks agreement because "a" indicates just one tree, while "trees" is plural. We must therefore change the phrase either to "a blue tree" or to "the blue trees."