Good explanation of how tenses are used

Using Other Tenses in Conjunction with Simple Tenses

(abstracted from http://owl.english.purdue.edu/owl/resource/601/08/)

It is not always easy (or especially helpful) to try to distinguish perfect and/or progressive tenses from simple ones in isolation, for example, the difference between simple past progressive ("She was eating an apple") and present perfect progressive ("She has been eating an apple"). Distinguishing these sentences in isolation is possible, but the differences between them make clear sense only in the context of other sentences since the time-distinctions suggested by different tenses are relative to the time frame implied by the verb tenses in surrounding sentences or clauses.

Example 1: Simple past narration with perfect and progressive elements
On the day in question...

By the time Tom noticed the doorbell, it had already rung three times. As usual, he had been listening to loud music on his stereo. He turned the stereo down and stood up to answer the door. An old man was standing on the steps. The man began to speak slowly, asking for directions.

In this example, the progressive verbs had been listening and was standing suggest action underway at the time some other action took place. The stereo-listening was underway when the doorbell rang. The standing on the steps was underway when the door was opened. The past perfect progressive verb had been listening suggests action that began in the time frame prior to the main narrative time frame and that was still underway as another action began.

If the primary narration is in the present tense, then the present progressive or present perfect progressive is used to indicate action that is or has been underway as some other action begins. This narrative style might be used to describe a scene from a novel, movie, or play, since action in fictional narratives is conventionally treated as always present. For example, we refer to the scene in Hamlet in which the prince first speaks (present) to the ghost of his dead father or the final scene in Spike Lee's Do the Right Thing, which takes place (present) the day after Mookie has smashed (present perfect) the pizzeria window. If the example narrative above were a scene in a play, movie, or novel, it might appear as follows.

Example 2: Simple present narration with perfect and progressive elements
In this scene...

By the time Tom notices the doorbell, it has already rung three times. As usual, he has been listening to loud music on his stereo. He turns the stereo down and stands up to answer the door. An old man is standing on the steps. The man begins to speak slowly, asking for directions.

In this example as in the first one, the progressive verbs has been listening and is standing indicate action underway as some other action takes place. The present perfect progressive verb has been listening suggests action that began in the time frame prior to the main narrative time frame and that is still underway as another action begins. The remaining tense relationships parallel those in the first example.

In all of these cases, the progressive or -ing part of the verb merely indicates ongoing action, that is, action underway as another action occurs. The general comments about tense relationships apply to simple and perfect tenses, regardless of whether there is a progressive element involved.

It is possible to imagine a narrative based on a future time frame as well, for example, the predictions of a psychic or futurist. If the example narrative above were spoken by a psychic, it might appear as follows.

Example 3: Simple future narration with perfect and progressive elements
Sometime in the future...

By the time Tom notices the doorbell, it will have already rung three times. As usual, he will have been listening to loud music on his stereo. He will turn the stereo down and will stand up to answer the door. An old man will be standing on the steps. The man will begin to speak slowly, asking for directions.

In this example as in the first two, the progressive verbs will have been listening and will be standing indicate ongoing action. The future perfect progressive verb will have been listening suggests action that will begin in the time frame prior to the main narrative time frame and that will still be underway when another action begins. The verb notices here is in present-tense form, but the rest of the sentence and the full context of the narrative cue us to understand that it refers to future time. The remaining tense relationships parallel those in the first two examples.

The present perfect is also used to narrate action that began in real life in the past but is not completed, that is, may continue or may be repeated in the present or future. For example: "I have run in four marathons" (implication: "so far... I may run in others"). This usage is distinct from the simple past, which is used for action that was completed in the past without possible continuation or repetition in the present or future. For example: "Before injuring my leg, I ran in four marathons" (implication: "My injury prevents me from running in any more marathons").

Time-orienting words and phrases like before, after, by the time, and others—when used to relate two or more actions in time—can be good indicators of the need for a perfect-tense verb in a sentence.

• By the time the Senator finished (past) his speech, the audience had lost (past perfect) interest.
• By the time the Senator finishes (present: habitual action) his speech, the audience has lost (present perfect) interest.
• By the time the Senator finishes (present: suggesting future time) his speech, the audience will have lost (future perfect) interest.
• After everyone had finished (past perfect) the main course, we offered (past) our guests dessert.
• After everyone has finished (present perfect) the main course, we offer (present: habitual action) our guests dessert.
• After everyone has finished (present perfect) the main course, we will offer (future: specific one-time action) our guests dessert.
• Long before the sun rose (past), the birds had arrived (past perfect) at the feeder.
• Long before the sun rises (present: habitual action), the birds have arrived (present perfect) at the feeder.
• Long before the sun rises (present: suggesting future time), the birds will have arrived (future perfect) at the feeder.