Tense: He walked (past), He walks (present), He will walk (future)

Tense–aspect–mood (commonly abbreviated tam) or tense–modality–aspect (abbreviated as tma) is a group of grammatical categories that are important to understanding spoken or written content, and which are marked in different ways by different languages.[1]

TAM covers the expression of three major components of words which lead to or assist in the correct understanding of the speaker's meaning:[2]

Tense—the position of the state or action in time, that is, whether it is in the past, present or future.
Aspect—the extension of the state or action in time, that is, whether it is unitary (perfective), continuous or repeated (imperfective).
Mood or Modality—the reality of the state or action, that is, whether it is actual (realis), a possibility or a necessity (irrealis).
For example, in English the word "walk" would be used in different ways for the different combinations of TAM:

Tense: He walked (past), He walks (present), He will walk (future).

Aspect: He walked (unitary), He was walking (continuous), He used to walk (repeated).
Mood: I can walk (possibility), Walk faster! (necessity).
In the last example, there is no difference in the articulation of the word, although it is being used in a different way, one for conveying information, the other for instructing.

In some languages, evidentiality (whether evidence exists for the statement, and if so what kind) and mirativity (surprise) may also be included. Therefore, some authors extend this term as tense–aspect–mood–evidentiality (tame in short).[3]The term was coined out of convenience[by whom?], for it is often difficult to untangle these features of a language. Several features (or categories) may be conveyed by a single grammatical construction (for instance, English -s is used for the third person singular present). However, this system may not be complete in that not all possible combinations may have an available construction. On the other hand, the same category may be expressed with multiple constructions. In other cases, there may not be delineated categories of tense and mood, or aspect and mood.

For instance, many Indo-European languages do not clearly distinguish tense from aspect.[4][5][6][7][8]

In some languages, such as Spanish and Modern Greek, the imperfective aspect is fused with the past tense in a form traditionally called the imperfect. Other languages with distinct past imperfectives include Latin and Persian.

In the traditional grammatical description of some languages, including English, many Romance languages, and Greek and Latin, "tense" or the equivalent term in that language refers to a set of inflected or periphrastic verb forms that express a combination of tense, aspect, and mood.

In Spanish, the simple conditional (Spanish: condicional simple) is classified as one of the simple tenses (Spanish: tiempos simples), but is named for the mood (conditional) that it expresses. In Ancient Greek, the perfect tense (Ancient Greek: χρόνος παρακείμενος, romanized: khrónos parakeímenos)[9] is a set of forms that express both present tense and perfect aspect (finite forms), or simply perfect aspect (non-finite forms).

However, not all languages conflate tense, aspect and mood. Some analytic languages such as creole languages have separate grammatical markers for tense, aspect, and/or mood, which comes close to the theoretical distinction.The English language allows a wide variety of expressions of combinations of tense, aspect, and mood, with a variety of grammatical constructions. These constructions involve pure morphological changes (suffixes and internal sound changes of the verb), conjugated compound verbs, and invariant auxiliaries. For English tam from the perspective of modality, see Palmer;[7] and Nielsen[24] for English tam from the perspective of tense, see Comrie[5] and Fleischman;[25] for English tam from the perspective of aspect, see Comrie.[6]

The unmarked verb

Edit
The unmarked verb form (as in run, feel) is the infinitive with the particle to omitted. It indicates nonpast tense with no modal implication. In an inherently stative verb such as feel, it can indicate present time (I feel well) or future in dependent clauses (I'll come tomorrow if I feel better). In an inherently non-stative verb such as run, the unmarked form can indicate gnomic or habitual situations (birds fly; I run every day) or scheduled futurity, often with a habitual reading (tomorrow I run the 100 metre race at 5:00; next month I run the 100 metre race every day). Non-stative verbs in unmarked form appearing in dependent clauses can indicate even unscheduled futurity (I'll feel better after I run tomorrow; I'll feel better if I run every day next month).

The unmarked verb is negated by preceding it with do/does not (I do not feel well, He does not run every day). Here do has no implication of emphasis, unlike the affirmative (I do feel better, I do run every day).

Morphological changes
Edit
The aspectually and modally unmarked past tense is usually marked for tense by the suffix -ed, pronounced as /t/, /d/, or /əd/ depending on the phonological context. However, over 400 verbs (including over 200 with distinct roots – short verbs for features of everyday life, of Germanic origin) are irregular and their morphological changes are internal (as in I take, I took). (See List of English irregular verbs.) This aspectually unmarked past tense form appears in innately stative verbs ("I felt bad.") and in non-stative verbs, in which case the aspect could be habitual ("I took one brownie every day last week.") or perfective ("I took a brownie yesterday."), but not progressive.

This form is negated with an invariant analytical construction using the morphologically unmarked verb (I / he did not feel bad, I did not take a brownie). As with do and do not, no emphasis is imparted by the use of did in combination with the negative not (compare the affirmative I / he did take the brownie, in which did conveys emphasis).

For the morphological changes associated with the subjunctive mood, see English subjunctive.There are two types of conjugated compound verbs in English, which can be combined.

The construction "have/has" + morphologically altered verb (usually with the suffix -"ed") indicates a combination of tense and aspect: For example, "have taken'" indicates a present viewing of a past or past and present event, so the continuing relevance of the event is an aspectual feature of the construction. The event in the past could be either stative, habitual, or perfective aspect, but not progressive.

Furthermore, the time of viewing itself can be placed in the past, by replacing have/has with had: had taken indicates a past viewing of an event prior to the viewing.
The have/has/had + verb + -ed construction is negated by inserting not between the two words of the affirmative (have/has/had not taken).
The construction "am/is/are" + verb + -"ing" indicates present tense combined with progressive aspect if the verb is innately non-stative, as in "I am taking it," or combined with innate stative aspect, as in "I am feeling better." For some stative verbs such as feel, the innate stative nature can be unmarked, so the simple verb form is used (I feel better), or it can be marked (as in I am feeling better) with no difference in meaning. For some other statives, however, either the unmarked form must be used (I know French, but not *I am knowing French) or the marked form must be used (I am experiencing boredom, I am basking in glory, but not I experience boredom or I bask in glory, which would imply a non-stative (specifically habitual) use of the verb).[26]
The am/is/are + verb + ing construction is negated by inserting not between the two words of the affirmative (am/is/are not taking).
The above two forms can be combined, to indicate a present or past view of a prior (or prior and current) event that occurred with stative or progressive aspect ("I have/had been feeling well," "I have/had been taking classes"); here the construction is "have/has/had" + "been" + main verb + -"ing". There is a subtle difference in usage between the case where the viewpoint is from the present and the case where it is from a point in the past: have been taking classes implies that the action is not only of present relevance but is continuing to occur; in contrast, had been taking classes indicates relevance at the reference time, and allows but does not require the action to still be occurring (I had been taking classes but was not still doing so; I had been taking classes and still was).
This combined form is negated by inserting not after have/has/had (I have not been taking classes).
Both of these morphological changes can be combined with the compound verbal constructions given below involving invariant auxiliaries, to form verb phrases such as will have been taking.