Tense, in grammar, a verbal category relating the time of a narrated event to the time of the speech event. In many languages the concept of time is expressed not by the verb but by other parts of speech (temporal adverbials or even nouns, for example).
Time is frequently perceived as a continuum with three main divisions: past, present, and future. The past and future times are defined in relation to the present time (now). Past tense refers to any time before the present time, and future tense refers to any time after the present. Not all languages perceive this relationship as a linear one, nor do these categories characterize all possible times. Tense, then, is a grammatical expression of time reference. The correlation between tense and time is not necessarily one-to-one; languages do not recognize as many oppositions of tense as they have conceptions of time. English has past, present, and future times, but only a past and a nonpast opposition of tense.
past: John ate lasagna.
present: John is eating lasagna.
future: John will eat lasagna.
Grammatical tense may not equal real time:
The flight is leaving at 5:00 pm.
That will be $5.00, please. [At a grocery checkout line.]
In the first sentence, the verb form that usually indicates present time is here used to indicate future time. In the second sentence, the verb form usually indicating future time is here used to indicate present time. The past form of the verb generally refers to past time, to a narrated event prior to the speech event.
In other languages the category of tense may express other oppositions, such as proximate versus nonproximate, now versus not now, etc. In English, however, the grammatical category of tense relates to the ontological concept of time in a binary opposition: past versus nonpast. Nonpast tense is considered “unmarked” for tense and thus can comprise present, future and even past times. With the exception of some problematic modal constructions—such as would in “John said he would go tomorrow,” in which would is grammatically a past tense of will but is used to indicate future time—the past tense indicates only past time and is thus said to be “marked” with respect to tense. Other grammatical categories, such as mood and aspect, may add another dimension to the time reference, further specifying the action as definite or indefinite, completed or not completed, lasting or nonlasting.