Rules for writers, Tenth edition - Diana Hacker, Nancy Sommers 2021

Use a linking verb between a subject and its complement
Sentence structure
Multilingual Writers and ESL Topics

Although their structure can vary, sentences in English generally flow from subject to verb to object or complement: Bears eat fish. This section focuses on the major challenges that multilingual students face when writing sentences in English. For more details on parts of speech and parts of sentences, see sections 47—50.

30a Use a linking verb between a subject and its complement.

Some languages, such as Russian and Turkish, do not use linking verbs (is, are, was, were) between subjects and complements (nouns or adjectives that rename or describe the subject). Every English sentence, however, must include a verb. For more on linking verbs, see 27e.