What is a tense?
In linguistic terminology, "tense" is a part of verbal paradigm that refers specifically to the time of an utterance. It is impossible for any language to have more than three tenses in this sense, since any action is either past, present, or future.
In English, we do the basic tenses this way:
- Present: I walk to the store.
- Past: I walked to the store.
- Future: I will walk to the store.
But what is that with the word will there in the future tense example? It turns out that while English can refer to present and past time using inflections on the verb itself, the future tense always requires another word. Furthermore, there are multiple ways of doing this:
- I will walk to the store.
- I'm going to walk to the store.
- I'm walking to the store in five minutes.
- I'm about to walk to the store.
So while English has plenty of ways to refer to future actions, in terms of base verbal morphology there are only two tenses in English: present and past.
So what about perfect, progressive, and the rest of that stuff?
Linguists refer to these as aspect. A verb's aspect refers to its duration, frequency, or completeness. English has three core aspects:
- Simple: I walk to the store.
- Progressive: I am walking to the store.
- Perfect: I have walked to the store.
Plus, we can combine progressive and perfect together as follows:
- Perfect progressive: I have been walking to the store.
Unfortunately, the way that these forms interact with meaning is very complex. In particular, we often use the simple present ("I walk to the store") to refer to habitual actions, and the simple progressive ("I am walking to the store") to refer to currently ongoing actions.
Now you've made me upset
That's because of mood, the other major component of the English verbal complex. Mood refers to the speaker's attitude towards the action, whether the speaker thinks the action is necessary, obligatory, inevitable, hypothetical, etc. We have a lot of moods in English, indicated by our modal verbs:
- I shall walk to the store.
- I will walk to the store.
- I should walk to the store.
- I would walk to the store.
- I may walk to the store.
- I might walk to the store.
- I must walk to the store.
- I can walk to the store.
- I could walk to the store.
Here, again, the form interacts with the meaning in a complicated way. The modal verbs will and shall tend to indicate future time more than anything really "moody", and there are constraints on which moods can be used in which tenses. Just to keep you on your toes.
Really we have 4 modal verbs which occur in present/past tense pairs: will/would, shall/should, can/could, may/might, and then must which can only be present-tense.
And don't forget about voice
Because we also have active voice and passive voice in English, which refer to the subject and the object are assigned to the verb.
- Active: I hit a dog with my car.
- Passive: A dog was hit by a car.
These have nothing to do with tense, but they are still part of the verbal paradigm.
Putting it all together
If you multiply all of those together you get eighty-eight possible combinations.
((5 moods × 2 tenses) + (1 defective mood)) × (4 aspects × 2 voices)
Don't try to memorize them all. Just try to remember the way the pieces interact, and you should be able to construct and interpret any combination that you come across. And remember that many verbs, like the past debitive perfect passive about to appear in this sentence, should rarely have ever been used by anyone.
But it's not that simple
It never is. The preceding elements are the core verbal paradigm, but there are a lot of other things that English does with its verbs to indicate elements of mood, aspect, or tense. Just to name two, we have:
- Past habitual: I used to walk to the store.
- Immediate future: I'm about to walk to the store.
There are lots of other combinations of helping verbs, adverbs, and prepositions which are sometimes used to express tense-like or aspect-like things in English. Merely knowing how the core verbal paradigm fits together doesn't necessarily help you interpret these kinds of utterances. Rather, these idiomatic verbal constructions have to be learned one at a time.
Also, please do read the comments on this answer, as the commenters have brought up numerous other subtleties and distinctions which I didn't get into the main post. The final takeaway of all this discussion is that English verbs are complex and you probably can't count how many forms they have.
Have fun!
Best Answer
I will interpret your question narrowly, that is, as really being about grammatical tense as opposed to other marked verbal1 systems such as aspect and mood. Even so, I will have quite a bit to say about aspect and mood.
1Verbal as in 'having to do with verbs'.
Answers to many questions will be more complicated or less complicated depending on whether you really have in mind the narrow technical sense of 'tense' or if you rather mean any marked verbal system (what some refer to as tense-aspect-mood). For example, in English, there are only four tenses in the strict sense (at least according to CGEL (p. 115)), but, by one count, there are as many as 88 'tenses' if the term is interpreted broadly.
The definition
Technically, grammatical tense is a verbal system
i. which is marked either inflectionally on the verb or analytically (e.g. by auxiliaries)
and where the basic or characteristic meaning of the terms
ii. is to locate the situation, or part of it, at some point or period of time
(CGEL, pp. 115-116).
As is often the case, however, in order to really explain a complicated concept, one must specify not only what it is, but also what it isn't. That is to say, one should explain what are some categories with which it might be confused. In the case of grammatical tense, there is possible confusion with other marked verbal systems, notably aspect and mood. Indeed, some people talk of tense–aspect–mood, since it is often difficult to untangle these features of a language.
Contrast with aspect and mood
Aspect and mood satisfy i. but replace ii. by something else. In the case of aspect, that something else is ii. 'has to do with the internal temporal constituency of the situation' (CGEL, p. 117). Another way to describe aspect is to say that it relates to 'other temporal information, such as duration, completion, or frequency, as it relates to the time of action. Thus, tense refers to temporally when while aspect refers to temporally how' (source).
CGEL takes the English aspect to contrast the progressive vs the non-progressive (p. 117):
About mood, CGEL says (p. 117)
Mood will be very relevant to the discussion of the (lack of a) future tense in English, below.
English tenses according to CGEL
CGEL says (p. 116) that English has two tense systems: the primary one, which contrasts the preterite and the present, and the secondary one, which contrasts the perfect and the non-perfect. All four combinations are possible:
preterite present
non-perfect went goes
perfect had gone have gone
To understand CGEL's reasoning in §6.3, we first need to explain several concepts.
In addition to Tr and To, there are two other 'times', of which we will only need one: the deictic time, Td. The word deictic comes from deixis, from the Ancient Greek word which means 'pointing, indicating, reference'. Deixis has to do with the 'reference point' from which the speaker 'points' to the things he or she is speaking about. Actually, the description of the deictic time will help with understanding of what deixis is about.
Finally, we are ready to explain...
Controversy 1: the perfect
Now the problem is that the division between tense, aspect, and mood is not always clear-cut. In English, one point of controversy is what to do with the perfect. In some languages, the perfect is clearly an aspect, whereas in English some authorities say it is an aspect, others say it is a tense, and still others say it is neither one of those but rather that it something called a phase (CGEL, p. 116). As I explained above, CGEL's take is that the contrast perfect vs. non-perfect constitutes a secondary tense system in English.
Controversy 2: the future tense
Another point of controversy in English is the future tense (or the lack thereof). This time the fuzzy boundary is between tense and mood, and CGEL, like many other authorities, says that the construction will/shall + plain form of the verb does not correspond to a future tense, but rather that will/shall are auxilliaries of mood, not tense. Here I will reproduce CGEL's argument in full (pp. 209-210):