There are in fact only two tenses in the English language (it's inportant not to confuse "tense" and "time" the two are not the same) if by tense you mean a change in word endings which is how linguists define "tense." English uses the "ed" ending to show the past and no ending to show the present.However, as noted above tense and time are not the same and as David Crystal writes in "Rediscover Grammar:"
"Present and past tenses can be used to refer to all parts of the time line:
THE MEANINGS OF THE PRESENT TENSE
Three present tense uses refer to present time.
1) The state present is used for timeless statements, or 'eternal truths':
Oil floats on water.
Two and two make four.
2) The habitual present is used for repeated events. There is usually an accompanying adverbial of frequency:
I go to town each week.
He drinks a lot.
The instantaneous present is used when the action begins and ends approximately at the moment of speach. It is very commonly used in demonstrations and sports commentaries:
Smith passes the ball....He shoots....
Three present tense uses refer to other times:
1) The historical present describes the past as if it were happening now:
I hear you've resigned.
2) In jokes and imaginative writing, a similar use promotes dramatic immediacy:
We look outside (dear reader) and we see an old man.
3) With some time adverbials the present tense helps to refer to a specific course of action in future time:
We leave tonight.
THE MEANINGS OF THE PAST TENSE
Most uses of the past tense refer to an action or state which has taken place in the past, at a definite time, with a gap between it's completion and the present moment. Specific events, states, and habitual actions can all be expressed using the past tense:
I arrived yesterday. (event)
They were upset. (state)
They went to work regularly (habitual)
The past tense used for the present or future.
1) The attitudinal past reflects the speaker's tentative state of mind, giving a more polite effect than would be obtained by using the present tense:
Did you want to go?
The hypothetical past expresses what is contrary to the speaker's beliefs. It is especially used in "if" clauses:
If you worked hard.....
I wish I had a bike.
3) In indirect speech a past tense used in the verb of 'saying' allows the verb in the subordinate clause to be past time as well, even though it refers to present time:
Did you say you had no money? (i.e. now)"
(David Crystal, 1996)
Many parts of the English verb which a lot of text books, and some grammar books, refer to as 'tenses' are in fact not. These are known as "aspects" and include the continuous, the perfect and the simple. The final part are the modals and they provide a phychological dimension which is called Grammatically "mood." (Note: There are other ways to construct "moods" eg the subjunctive, but the modals are the most common).
What about the futue? How can English refer to the future without a futue tense? In fact English does not have a future tense and I have seen both teachers and text book writers getting into massive trouble by insisting it does, I am looking at the moment at a poster published in Poland where the publishers have had to invent a "fourth tense" and been forced to call it "Future in the past."
In fact English used a number of different ways to refer to the future, sometimes using the present tense, with both simple, contininuous and perfect aspects, and sometimes the modals eg:
I will be there tomorrow.
I might be there tomorrow.
I must be there tomorrow.
I would be there tomorrow but I have to go to Paris instead.
English verb tenses are not easy, but neither are they as difficult as most people think the main point is to understand that time is not the same as tense and the student should not be taught as if it were.
Just to summarise, We are left therefore with the following:
Note: "Dare (to), ought (to) and others are still under debate.
The English verb system is a combination of these factors each of which brings its own ideas into the final meaning of the verb. Hope this helps.