How do we express time in English? How do we indicate that an action happened in the past, or is happening now or is expected to happen in the future?

Grammatically speaking, we express time using a combination of tense and aspect, supported at times by phrases such as 'yesterday' and 'tomorrow night'. In this note on grammar, we will look briefly at the concepts of tense and aspect, and outline the tenses of the English verb. The purpose is to gain a basic understanding of how English is put together, so that we can analyse our own writing and improve it.

The Oxford Reference Grammar explains that tense "refers to the location of a situation in time"(11.4). This location is expressed in English in the form of the verb:

Simple present: I, you, we and they laugh; He or she laughs
Simple past: I, you, he, she, we and they laughed

You can see that the simple present uses two forms of the verb: laugh (the 'base' form of the verb) and laughs. The suffix -s (-es where the verb already ends in a sibilant) is used for the third person singular in all verbs except the modals ('can', 'must', 'shall', 'will', 'should' and 'may').

The simple past uses one form of the verb: laughed, where -ed (or -d if the verb already ends in e) is added to the base. However, there are common exceptions to this, such as (I, you, he, she, we and they) wrote, ran, and slept.

Aspect, on the other hand, "refers primarily to the way that the time of the situation is regarded rather than its location in time in absolute terms"(11.4). It is expressed by the use of an auxiliary verb with the main verb. In English, there are two aspects: one where the action is considered as continuing (progressive aspect), and one where the action is considered complete (perfect aspect).

Progressive aspect: I am laughing
Perfect aspect: I have laughed

Some grammarians regard the perfect aspect as a tense because it refers to an action in the past, either of the speaker or in real time; for example:

I have taken out the rubbish, just as you asked me to.

Grammatically, this sentence is in the present (tense) perfect (aspect), but the action of taking out the rubbish can be placed in the past and we would therefore naturally think of it as being in the past tense. However, other grammarians see the perfect as an aspect "because it is retrospective in at least some of its uses"(11.4). For example:

I have always thought him unforgiving.

This sentence also is in the present perfect. But, here, the speaker is making a retrospective observation. The intention is to underscore that those impressions, from the point of view of the speaker, were held prior to now and are still held now.

The purpose here is not to get into the details of these issues, but to point out that the two concepts of tense and aspect are not clear cut.

From a strictly grammatical point of view, in which tense is defined as being shown by a verb inflection, the present and the past are the only two tenses in English. The future is not regarded as a tense, because it is expressed using an auxiliary verb (usually, 'will'). In fact, auxiliary verbs enable many categories of time to be expressed, such as the future perfect and past progressive. The use of these auxiliaries will be covered in future notes on grammar.

Reference: Sidney Greenbaum: The Oxford Reference Grammar, edited by Edmond Weiner, Oxford University Press, 2000.