As you know, in Modern English adjectives hardly ever change their form to "agree" with the thing they're modifying."tall tree" and "tall trees" "bad boys" and "bad girls"Old English adjectives are different, and must change endings to show the different numbers, genders, and cases of the nouns they modify. But look again at the adjectives "this" and "that". When the nouns they modify become plural, the adjective itself changes form: from "this" to "these"; from "that" to "those". These two are the only adjectives in Modern English which change their forms to match a grammatical feature of the nouns they're modifying. They have slightly different forms to indicate a change in number of the nouns they modify.So, these words are adjectives, since they qualify nouns, and since their main purpose is to "point out" the nouns, we call them "demonstrative adjectives" because they "point out" or "point to" (Latin "demonstrare"). This is very important to remember: these words are "demonstrative adjectives".
