An adjective can be defined as a word that modifies a substantive (a noun or pronoun). Such a word can be used either before the noun (attributive position—”an old man”), or after a linking verb (predicate position—”The man is old”).
Comparing adjectives means showing the degree to which nouns exhibit a given quality relative to each other. There are three degrees of comparison: positive, comparative, and superlative. The positive degree is the dictionary form of the adjective. Comparative adjectives are used when two nouns are compared; superlative adjectives are used when more than two are compared.
Some words are classed as adjectives in that they modify nouns, but they cannot show degrees of comparison. Here is a list of adjectives that fit that description: stone, brick, wooden (and other words that describe what something is made of); baseball (or any sport word used as modifier), proper adjectives (e.g., those describing nationality).
Adjectives and adverbs are similar, and can be confused with each other. Both are modifying words, and both can show degrees of comparison. Both adjectives and adverbs can be used as complements of the verb “to be.” However, an adverb modifies a verb, adjective, or other adverb, and typically ends in -ly (though this ending can also be used to make an adjective from a noun). A linking verb—such as “seem,” “smell,” or “look”—is followed by an adjective modifying the subject, not by an adverb (“This milk smells sour”).
Many other languages display agreement. This means that an adjective must be plural if the noun it modifies is plural (French “la belle fille—les belles filles” the beautiful girl/girls”). If nouns in the language have gender and/or case, the adjective too must be inflected accordingly.
In some other languages, the category of adjective is not as well developed as it is in English. The Hebrew of the Bible, for instance, prefers either (1) possessive constructions (“mountain of holiness” rather than “holy mountain”), or (2) stative verbs (verbs that describe a state of being—as if we said “He olded” instead of “he was old”).