An author keeps the time consistent in his writing by his control over the verb. The verb not only plays an important part in setting the time, but also it must agree with the subject in both number and person. The basic tenses are past, present, and future, but there is a wide range of intermediate tenses which help the writer to specify the time in various ways. He may specify whether an action is in progress at the moment, whether it has been continuing over a period of time or has just occurred occasionally, and whether one action in the past happened before another action in the past. The table below applies to the regular verbs and will give you an idea of the many shades of meaning that are made possible by the various tenses.
The present tense is used (among other functions) in command, suggestions, and to indicate habitual action or continuing ability.
The future tense signals action that is to take place in the future at a time which may or may not be specified. It is formed by will plus a verb in the present tense.
The past tense is formed by adding d or ed to the present tense of the verb and signifies an action that has been completed.
The two verbs most commonly used as auxiliaries are have and be. The auxiliary will signals future action. Progressive tenses can be identified by the ing ending.
The verb forms were and be are used to express wishes or hopes, orders and statements contrary to fact.
Be is sometimes used as subjunctive when one wishes to express a command or a desire. It is usually found in subordinate clauses introduced by that.