Here is a brief explanation of each of the Greek verb tenses:

1.   Present Tense

In English, we know that the present tense describes something happening right now.  It informs us of the time when an action takes place.

In Greek, however, the present tense primarily tells us the type of action.  The Greek present tense indicates continued action, something that happens continually or repeatedly, or something that is in the process of happening.  If you say, for instance, "The sun is rising," you are talking about a process happening over a period of time, not an instantaneous event. The Greeks use the present tense to express this kind of continued action.

In contrast, Greek uses the aorist tense to show simple action.  An aorist verb simply tells you that something happened, with no indication of how long it took.  Aorist is like a snapshot; present is like a video.

When the verb in question is in the imperative, subjunctive, or optative mood, or is an infinitive, present tense says nothing at all about the time when an action takes place.  It does not mean that something is happening right now.  Its only significance is to show that the action happens continuously or repeatedly.  In Ephesians 5:18, for example, Paul uses a present imperative when he tells believers to "be filled with the Spirit."  The present tense makes it clear that this is a continuing experience, which they should maintain constantly.

We face a slightly different situation when we deal with verbs in the indicative mood, the verb form used for statements of fact.  Indicative verbs bear a double burden: they must reveal the time of an action, not just the type of action.  A present indicative verb describes an action taking place at the present time.  Normally, this action is a continued action taking place right now.

However, that is not always the case.  Suppose a Greek writer wants to describe a balloon that pops right now!  He will have to use the present tense, even though the balloon burst takes only a moment.  There is no process; it doesn't happen gradually.  It would be nice if you could use the aorist tense to describe it, but that won't work because the balloon didn't pop yesterday -- and aorist indicative verbs can only describe the past.  

Acts 9:34 is a similar situation.  Peter has been called to the bedside of Aeneas, who has been paralyzed for eight years.  The apostle announces, "Aeneas, Jesus Christ heals you!"  And the word "heals" is present tense. Is Peter describing a long process of healing that gradually begins to happen?  Not this time.  Healing happens instantaneously; the next clause says, "And immediately he arose."

Summary:  The Greek present tense usually describes action that is in the process of happening, or action that continues over a period of time.  In the indicative mood, however, it can refer to other types of action.

2.  Aorist Tense

 The aorist tense is the Greek grammarian's term for a simple past tense.  Unlike the other past tenses (imperfect and perfect), the aorist simply states the fact that an action has happened.  It gives no information on how long it took, or whether the results are still in effect.

Aorist is an ideal tense to describe an action that happens at a particular point in time.  This is why some grammar books describe it as "punctiliar."  Aorist verbs describe the entire action as a single event.

This does not mean that aorist tense always describes actions which are over in an instant.  John 2:20 says, "This temple was built (aorist) in forty-six years.  Forty-six years is a big point in time!

When you find the aorist tense in the indicative mood, it routinely communicates two ideas:  (1) past tense and (2) simple type of action.

When you encounter an aorist participle, it still describes a simple action, but it may not refer to action in the past.  Most often, an aorist participle describes an action that takes place before the main verb of the sentence. [There are exceptions to this rule.]

When you see an aorist anywhere other than indicative or participle, there is no time frame involved.  You should focus exclusively on the type of action: simply the fact that an event happened, with no extra information about continuing action or completed action.

Aorist is the default tense, especially when a writer is describing the past.  If in doubt, Greeks would use aorist.  If you see any other tense, you should suspect that it was deliberately to make a point.

3.  Imperfect Tense

Students just learning Greek usually like the imperfect tense, because it only occurs in the indicative mood - no participles, no infinitives, no imperatives.  And anyone who is seriously trying to master the language is always grateful for something that they don't have to learn!

The meaning of the imperfect tense is straightforward:

Past time - Imperfect always describes something that happens in the past.

Continued action - Imperfect always describes something that is continued, repeated or habitual.  There are some minor refinements of this explanation, but it is almost always safe to view an imperfect verb as continued action, not the simple action of the aorist or the completed action of the perfect tense.

When you encounter an imperfect verb in Greek, imagine that you have been zapped in a time machine and dumped into a scene in the past.  You look around and ask, "What's going on?" Maybe you're standing in a French village in the Dark Ages, and all you see are peasants chopping down trees.  You don't know when they started their task, and you don't know how long they will keep it up.  You just know that the work was in process when you looked.

That's when Greek uses the imperfect tense.  When you return to the 21st century to give your report, you say, "The peasants were chopping down trees."

Summary:  The imperfect tense is the perfect way to describe an action that was in the process of happening at some time in the past.

4.  Future Tense

Greek has three tenses that describe the past:  aorist, imperfect, and perfect.  This allows a Greek writer to be specific about the three different types of action that can come into play: simple, continued, and completed.  

But there is only one future tense, and the Greeks had to use it to cover all the possible types of action. It is probably best to assume that most future tense verbs are describing simple actions, without including extra concepts like continued action.  This is not an area where you should build elaborate arguments on the grammar.

What can we say about a future tense verb?  Grammarians have rightly pointed out that there are at least two shades of meaning which can be conveyed by a future tense verb:

1.  Prediction -- Most future tense verbs are simple predictions of what will happen.

Example:  "It will rain tomorrow."

Biblical example:  Matthew 1:21 - "And she shall bear a Son."

As we read the New Testament, it is important to remember that a prediction is only as good as the person who does the predicting.  When God says something is going to happen, you can count on it.  But when the Pharisees predict something, you might wat to get a second opinion!

2.  Command -- Occasionally a future tense verb is actually a command or instruction.

Example:  "You will clean your room!"

Biblical example:  Matthew 19:18 - "You shall not commit murder."

How can you tell which idea is in effect for a given verse?  There is no difference in the spelling or forms of the word, so you must examine the context and use your common sense to determine which idea makes the best sense in that passage.

5.  Perfect Tense

The perfect tense in Greek is used to describe a completed action which produced results which are still in effect all the way up to the present.  Sample translation:  "I have believed."

Notice that the perfect tense carries two ideas:  (1) completed action and (2) continuing results.  The action was completed at some time in the past, and the results continue up to the present.

Example:  We can see the perfect tense in action in 1 John 1:3:  "What we have seen and [have] heard we proclaim to you also."

The apostle John is making the point that he was an eyewitness to the earthly ministry of Jesus Christ, and that personal experience serves as the basis for the message that he proclaims decades later.  You might paraphrase the first verb as "We saw Him, and we can still visualize what we saw."  One writer has explained the second verb as "We heard Him, and His words are still ringing in our ears."  John saw and heard Jesus many years earlier, and that era of his life has been completed.  But the results continue.  What he learned so many years ago remains with him now.

6.  Pluperfect Tense

The pluperfect is a seldom-used tense related to the perfect.  It occurs only 86 times in the New Testament, and most Greek teachers spend little time on it.  But to be complete, here is an explanation:

The pluperfect has the same meaning as the perfect tense, except that it only brings the results of an action up to a selected time in the past.  The perfect tense, in contrast, brings the results all the way up to the present.

While perfect tense is usually translated "I have believed," pluperfect is translated "I had believed." If I want to tell you that I have memorized the Greek alphabet and I still remember it well enough to pass a quiz today, the perfect tense is the best choice to use.  On the other hand, suppose I have not studied Greek recently. I probably could not pass a quiz today, but I got a really good score on the quiz I took last month.  The pluperfect is the tense of choice for that idea:  "When I took the quiz last month, I had learned the Greek alphabet perfectly."