i-adjectives are considered a special closed-class of verbs because they can conjugate directly. Because they can conjugate to demonstrate aspect, you cannot use "da" with them. However, they cannot convey politeness on their own, so you can use "desu" after them.
i-adjectives, in their present tense, always end with the hiragana い. The only adjectives that end in い and are not i-adjectives are きれい ("kirei", clean) and 嫌い ("kirai", hate).