Verbs agree with their subjects in person and number. We all know this but we do not always remember it. Unless you are very careful, you will find yourself using a singular subject with a plural verb or the reverse. Mistakes of this sort are particularly liable to happen in the case of collective nouns, in the use of personal pronouns as subjects, and in cases where the subject and the verb are far separated in the sentence.
Those forms of the verb which tell whether the subject is acting or is acted upon are called voices. When the subject is acting the verb is said to be in the active voice. When the subject is acted upon the verb is said to be in the passive voice. Verbs in the passive voice have no objects because the subject, being acted upon, is itself in the place of an object.
Those forms of the verb which tell whether the time of the action is past, present, or future, are called tenses. They are six, viz.
Present, I print (am printing) the book.
Past or imperfect, I printed the book.
Future, I shall print the book.
Perfect, or present perfect, I have printed the book.
Pluperfect or past perfect, I had printed the book before you wrote.
Future perfect, I will notify you when I shall have printed the book.
When adverbs denoting time are indicated care should be taken to see that the verb is consistent with the adverb. "I printed it yesterday," not "I have printed it yesterday;" "I have not yet printed it," not "I did not print it yet;" "I have printed it already," not "I printed it already."
Trouble is sometimes found in choosing the right forms of the verb to be used in subordinate clauses. The rule is:
Verbs in subordinate sentences and clauses must be governed by the tense of the principal verb.
This rule rests on the exact meaning of the forms and words used and its application can be checked by careful examination of these meanings. "He said he did it." "He said he would do it." "He says he will do it."
Note that when the statement in the subordinate clause is of universal application the present tense is always used whatever the tense of the principal verb. "The lecturer said that warm weather always softens rollers."
Those forms of the verb which tell whether the action is an actual fact, a possibility, a condition, or a command are called moods.