Some languages do not require a subject in every sentence. Every English sentence, however, needs a subject.
EXCEPTION: In commands, the subject you is understood but not present in the sentence: Give me the book.
The word it may be used as the subject of a sentence describing the weather or temperature, stating the time, indicating distance, or suggesting an environmental fact.
Do not omit it in such sentences.
It is raining in the valley and snowing in the mountains.
It is 9:15 a.m.
It is three hundred miles to Chicago.
In July, it is very hot in Arizona.
In some English sentences, the subject comes after the verb, and a placeholder called an expletive (there or it) comes before the verb.
English does not allow a subject to be repeated in its own clause.
Do not add a pronoun even when a word group comes between the subject and the verb.
Do not repeat an object or an adverb in an adjective clause. Adjective clauses begin with relative pronouns (who, whom, whose, which, that) or relative adverbs (when, where). Relative pronouns usually serve as subjects or objects in the clauses they introduce; another word in the clause cannot serve the same function.
If the clause begins with a relative adverb, do not use another adverb with the same meaning later in the clause.