1. The word it is often used in an indefinite way at the beginning of a sentence: [It snows]. When so used, it has no antecedent, and we say it is used impersonally.
2. The pronoun it is often used as the grammatical subject of a sentence in which the logical subject is found after the predicate verb: [It is impossible for us to go]. When so used the pronoun it is called an expletive. There is used in the same way.
+34. Cautions and Suggestions.+
1. Be careful not to use the apostrophe in the possessive forms its, yours, ours, and theirs.
2. Be careful to use the nominative form of a pronoun used as an attribute complement: [It is I; it is they].
3. Be sure that the pronoun agrees in number with its antecedent. One of the most common violations of this rule is in using their in such sentences as the following:—Every boy and girl must arrange his desk. Who has lost his book? The use of every and the form has obliges us to make the possessive pronouns singular.
His may be regarded as applying to females as well as males, where it is convenient not to use the expression his or her.
4. The so-called subject of an infinitive is always in the objective case: [I asked him to go].
5. The attribute complement will agree in case with the subject of the verb. Hence the attribute complement of an infinitive is in the objective case: [I knew it (obj.) to be him]; but the attribute complement of the subject of a finite verb is in the nominative case: [I knew it (nom.) was he].
6. Words should be so arranged in a sentence that there will be no doubt in the mind concerning the antecedent of the pronoun.