Some of the terms used to describe English grammar are more relevant to the description of Latin grammar, and these include those that describe certain verb forms. According to the traditional nomenclature, the Latin verb had six active indicative tenses (present, imperfect, future, perfect, pluperfect and future perfect) and four active subjunctive ones (the indicative ones minus future and future perfect). Each of these forms was indicated by a change in the shape of the finite verb. (The shape of a word is the way it’s spelled or pronounced, rather like its ‘morphology’.) For example, where the present indicative tense of the verb meaning ‘he walks’ was ‘ambulat’, in the future it was ‘ambulabit’ and in the imperfect it was ‘ambulabat’.
‘Tense’ characterizes the way in which a verb’s shape changes to indicate the time at which the event the verb describes takes place. That is exactly what it does in Latin. The English verb does so too, but only in two forms, past and present. ‘Walks’ is present tense and ‘walked’ is past tense. This has always been the case. The Old English verb had subjunctive forms as well as indicative forms, but, like Modern English, had only two forms, present and past, to indicate time by changes in the verb’s shape.
English has several other verb forms that do not depend on changes in the shape of its finite forms. The past participle (‘walked’) and the ‘-ing’ form (‘walking’) can combine with ‘be’ and ‘have’ to produce:
(1) He is walking
(2) He has walked
(3) He had walked
(4) He has been walking
(5) He had been walking
They can also combine with ‘will’, as well as ‘be’ and ‘have’, to produce
(6) He will walk
(7) He will be walking
(8) He will have been walking
(9) He will have walked
These are clearly different types of verb form from ‘walks’ and ‘walked’ and it is unhelpful to use the same term, ‘tense’, to describe them. In each of those sentences there is only one finite verb and it is neither ‘walking’ nor ‘walked’. It is the present tense of ‘be’ in (1), the present tense of ‘have’ in (2) and (4) and the past tense of ‘have’ in (3) and (5). In (6), (7), (8) and (9) it is ‘will’, in so far as a modal verb can be considered to be finite.
These forms express aspect, that is, the view that the speaker takes of the event described by the verb. The aspect expressed in (1) and (7) is progressive, in (2), (3) and (9) perfective, and in (4) (5) and (8) perfective progressive. The tenses of the verb also express aspect. The present tense (‘walks’) expresses habitual aspect, as does (6), and the past tense (‘walked’) expresses perfect (not to be mistaken for perfective) aspect.
English has no future tense, but can nevertheless express the future in a number of ways. The present tense can be used to form what is sometimes informally known as the ‘diary future’, as in ‘I fly to New York on Saturday.’ Alternatively, the ‘-ing’ form of the main verb can combine with ‘be’ to produce ‘I’m flying to New York on Saturday’, or the base form can combine with the modal verb ‘will’ to produce ‘I’ll fly to New York on Saturday’, or it can combine with ‘going to’, to produce ‘I’m going to fly to New York on Saturday’.