I have asked these questions of many students over the years. By far the most common answers are that tense has something to do with the time of the sentence and that there are three tenses: past, present, and future. Some people, perhaps remembering their foreign-language classes, will list more tenses, with names like pluperfect and so on. Some grammar books have long lists of inflections of verbs with names like the past perfect tense (for example, "had played"), or the future progressive tense (for example, "will be playing").
If you never could keep all these straight, you are not alone. One reason you may have problems is that the story that most schoolbook grammars tell about tense is not particularly accurate. These books are frequently vague about just what tense is, and they implicitly lump together separate elements of the verb phrase into this single category. One consequence of this muddled pedagogy is that students come away with the sense that anything having to do with the verb should be called a tense. It is easy, for example, to find instances of journalists or other educated people talking about the "passive tense" (it's actually a voice, as we will see in a later chapter).
Before I reveal how we will actually treat tense, I would like to step you through a short exercise that will show some of the problems with the traditional conception of tense. To begin, fill in the sentence "Marissa ________ her dog" with the form of the verb walk that is appropriate for each of the three primary tenses that you were taught: past, present, and future. Write these down so you will have something to refer to as you look at the next set of examples.
Form used in the present tense: ________________
Form used in the past tense: ________________
Form used in the future tense: ________________
Pay attention in particular to what distinguishes one form of the verb from another. (Note that the form of the present-tense verb that you wrote could have been different if we had used a different subject, for example, they. This difference is separate from tense, and so to keep things simple, all of the examples that follow will use will employ similar subjects so that we only need to consider one form for each tense.)
Now consider the following sentences. For each one, look at the underlined verb. What tense does each one have? Don't be distracted by the meaning of the sentence. Just look at the form to answer this.
(2) My flight leaves at 10 pm.
(3) Marissa walks her dog each evening.
(4) Your mother tells me you plan to go to law school.
(5) Sherry will be sorry that she missed seeing you this evening.
(6) If he studied, he could pass the upcoming test.
Now look at the time of the action to which each verb refers. Do you see the problem?
In sentence (2), you may have been tempted to declare leaves a future-tense verb, but compare the form to our previous list. It is actually a present-tense form, although the sentence refers to a future event. In sentence (3), walks is a present-tense verb, but notice that the time it describes is not really now. This statement can be true even if the dog-walking is not occurring at the moment of the statement, for example if it's morning. Sentence (4) also contains a present-tense verb, tells, but the act of telling clearly took place before the statement, and so refers to past-time. In sentence (5), missed is in the past tense, but notice that this event (the missing) is ongoing during the time that the sentence is being uttered. From the frame of the speaker, it occurs in the present time. In sentence (6), the proposed action (studying), along with the test, lies in the future, but studied is a past-tense form.
What is going on here?
These examples illustrate that tense does not always equate simply with time. When we use the term tense, we are referring to a grammatical form. Time, however, is a semantic concept that can be expressed in ways other than a grammatical marking of the verb. In sentence (2), for example, the futurity of the action is conveyed not by the verb but by the prepositional phrase at 10 pm. Further, tense can be used, in extended senses, to convey meanings other than time. In sentence (6), the past tense marks not past time but the speaker's opinion that the subject is unlikely to actually study and that the situation is therefore a hypothetical one.
Once we appreciate this crucial distinction between form and meaning, we are ready to look at exactly what tense is. As we will define it, tense refers to a grammatical form, or system of forms, whose primary function is to refer to a point in time.
This definition of tense is narrower than the one typically given in schoolbooks. Note in particular that while pointing to a time is the primary function of tense, it is not the only function. Further, this function doesn't involve every possible aspect of time, only reference to basic points in time. As we will discover shortly, there are other features of a temporal situation that are conveyed with different means.
How many tenses does English have? By now, I hope I have convinced you to mistrust the simple explanations of the schoolbooks. Let's return to the examples of the basic tenses that we produced before:
Tense according to the schoolbooks:
Looking at these forms, the future seems very different While the present and the past are formed synthetically, that is by means of an inflection, the future is formed analytically, that is by means of an auxiliary verb. By itself, that difference may not be decisive—the comparative degree of adjectives, for example, can be expressed either synthetically (quieter) or analytically (more pleasant)—but enough differences distinguish the traditional future tense from the present and past tense forms that it does not make much sense to lump them together.
First, in terms of grammatical structure, will is not unique. It operates like many other auxiliary verbs, verbs which are sometimes called conditionals, but which we will call modal verbs. Examples of other modal verbs are can, may, should, or must. These verbs will be the subject of the next section, but for now notice that each of these combines with another verb in exactly the same way: the auxiliary is followed by the bare form of the verb:
(7a) Marissa will walk her dog.
(7b) Marissa can walk her dog.
(7c) Marissa may walk her dog.
(7d) Marissa should walk her dog.
(7e) Marissa must walk her dog.
In terms of the semantics, there are various shades of meaning conveyed by the different modal verbs. Sentences 7a-e differ in the degrees of possibility or obligation that they express, but all of these sentences refer in some way to an event that has not yet occurred. In other words, the situation is located in the future. Thus will is not unique in picking out a future time. Moreover, there are some contexts in which will is not the normal way we refer to a future action. For example, suppose you have plans to go to a party tomorrow, and a friend asks you to see a movie with her. Which response would be normal to decline that invitation?
(8a) Sorry, I will go to the party.
(8b) Sorry, I'm going to the party.
Sentence (8b), of course, would be the normal response. English speakers regularly use the second form to refer to future action when there is a definite plan. Indeed, if we think about the contexts in which (8a) might be acceptable, we can see that (8a) expresses more than just the future time of an event. It also conveys the speaker's firm determination. You might say it, for example, in response to someone who has told you that you should stay home and study. ("Sorry, I WILL go to the party.") This additional element, telling us something about the speaker's attitude in addition to the time, is frequently conveyed by other modal auxiliaries.
(9) She must have been drunk.
As in (8a), sentence (9) expresses a conclusion about the speaker's attitude or understanding of a situation. As we will see shortly, expressing this sort of meaning is one of the common functions of modal auxiliaries.
Finally, and perhaps most strikingly, in sentences with multiple verbs, will appears in contexts with present-tense verbs. Conversely, the closely related would appears in contexts with past-tense verbs.
(10a) Scientists predict that the volcano, which has been inactive for many years, will erupt at any moment.
(10b) Scientists predicted that the volcano, which had been inactive for many years, would erupt at any moment.
Notice that the highlighted verbs in (10a) are present tense, and the highlighted verbs in (10b) are past tense. Moreover, we cannot substitute would for will or vice versa.
(10c) *Scientists predict that the volcano, which has been inactive for many years, would erupt at any moment.
(10d) *Scientists predicted that the volcano, which had been inactive for many years, will erupt at any moment.
Sentences (10a) and (10b) illustrate the tendency of tense consistency. In other words, unless there is some overriding reason to switch tenses, the basic tense of a sentence will remain consistent throughout. In short, will is consistent with present-tense verbs and inconsistent with past-tense verbs.
Taken together, all these observations lead to a surprising conclusion: English does not have a future tense. English tenses are expressed by inflections on the verb. That means that English has only two tenses: present and past. Will is an auxiliary and part of a different verbal system, that of mood. Will does have a tense, but as examples 10a-d show, it is a present-tense verb.
This conclusion differs dramatically from what is typically taught in schoolbook grammars, but it is not new-fangled linguistics. The two-tense nature of English, and of other Germanic languages, was first recognized in the early nineteenth century, and is currently the standard account in the reference works used by professional linguists. That so many books used in primary and secondary education still cling to an outdated description is scandalous but unfortunately typical of the disconnect between the authors of such books and linguistic scholarship.
 As far as I know, no grammar book actually calls the passive voice a tense. The problem, in this instance, is not with the actual labels used but with the failure to teach how the overall system actually works in a way that students retain.
 English is classified as a Germanic language because, despite heavy later borrowings of French, Latin, and Greek words, its core words and grammar are most closely related to languages like German, Dutch, Swedish, etc., all of which belong to the Germanic family of languages.