There are three types of sentences.

(A) 犬は動物だ。 Dogs are animals.
(B) ひろ子はやさしい。 Hiroko is kind.
(C) さやかはすぐ怒る。 Sayaka often gets angry.

The sentences (A), (B), and (C) are the three basic types of Japanese sentences, which are called the noun-predicate, the adjective-predicate, and the verb-predicate respectively. You need to use the auxiliary verb だ at the end of noun-predicates to assert the whole sentence. You don't need to use the auxiliary verb だ at the end of adjective-predicates or verb-predicates because adjectives and verbs have the function of assertion by themselves.


Sentence Structures