Wikipedia
Tense–aspect–mood, commonly abbreviated and also called tense–modality–aspect or , is the grammatical system of a language that covers the expression of tense (location in time), aspect (fabric of time – a single block of time, continuous flow of time, or repetitive occurrence), and mood or modality (degree of necessity, obligation, probability, ability). In some cases, evidentiality (whether evidence exists for the statement, and if so what kind) may also be included.
The term is convenient because it is often difficult to untangle these features of a language. Often any two of tense, aspect, and mood (or all three) may be conveyed by a single grammatical construction, but this system may not be complete in that not all possible combinations may have an available construction. In other cases there may not be clearly delineated categories of tense and mood, or aspect and mood.
For instance, many Indo-European languages do not clearly distinguish tense from aspect. 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 is classified as one of the simple tenses , but is named for the mood (conditional) that it expresses. In Ancient Greek, the perfect tense is a set of forms that express both present tense and perfect aspect (finite forms), or simply perfect aspect (non-finite forms).
Not all languages conflate tense, aspect, and mood, however; close to a theoretically ideal distinction, with separate grammatical markers for tense, aspect, and/or mood, is made in some analytic languages such as creole languages.