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

Include a subject in every sentence
Sentence structure
Multilingual Writers and ESL Topics

Some languages, such as Spanish and Japanese, do not require a subject in every sentence. Every English sentence, however, needs a subject.

Commands are an exception: The subject you is understood but not present in the sentence ([You] Give me the book).

The word it is used as the subject of a sentence describing the weather or temperature, stating the time, indicating distance, or suggesting an environmental fact.

In most English sentences, the subject appears before the verb. Some sentences, however, are inverted: The subject comes after the verb. In these sentences, a placeholder called an expletive (there or it) often comes before the verb.

Sometimes an inverted sentence has an infinitive (to work) or a noun clause (that she is intelligent) as the subject. In such sentences, the placeholder it is needed before the verb. (See also 49b and 49e.)

NOTE: The words here and there can be used as placeholders. When they mean “in this place” (here) or “in that place” (there), they are adverbs, which are never subjects.