Tenses in English grammar are used to indicate distinctions of time. For instance, while verbs do indicate when actions have taken place, there are three tenses in English which indicate actions as having occurred in the past. Yet, they differ in their indication of the point in the past at which these actions took place, and there are differing types of these actions.
In the Basic Form in which an action is begun and completed in the past, there are two tenses that define the distinction of past time. The simple (meaning one word) Past Tense indicates an action that was begun and completed in the past. e.g. I finished my paper last evening. If, however, one wishes to indicate that he/she has completed an action in the past before another past action, the Past Perfect tense is employed. e.g. I had eaten supper before I finished my paper last evening.
There is another distinction that can be made which bridges the present time with the past. This tense is the Present Perfect Tense. If, for example, a person wishes to indicate that he/she has done something in the past but this action has continued into the present, this tense is employed. e.g. I have finished my paper this evening. (Perhaps, the person has been working on this assignment for days and is now finished "this evening.")
In addition to these Basic Forms of the three tenses, there is the Progressive Form which sets the verbs in the past, but without a definitive time of completion. Usually this action was in progress/going on when another action occurred. Here are examples of these three tenses in the Progressive Form:
Past Progressive: I was finishing my paper when the electricity went out. (This action was going on in the past as another action occurred.)
Past Perfect Progressive: I had been working on my paper, but fell ill and was not able to complete it until days later. (This action was in progress in the past, but another action occurred after it.)
Present Perfect Progressive: I have been working on my paper now for seven days. (This action was begun in the past and is continuing now in the present.)