The terms tense, aspect, and modality refer to three kinds of information that are often encoded by verbal morphology. tense marking indicates, to varying degrees of precision, the time when an event occurred or a situation existed. In other words, it specifies the situation's “location” in time. aspect relates to the distribution of an event over time: is it instantaneous or a long, slow process?; completed or ongoing?; once only or a recurring event?

modality covers a wide range of semantic distinctions, but generally relates to either the speaker's attitude toward the proposition being expressed (e.g. his degree of certainty about whether the proposition is true or not), or the actor's relationship to the described situation (e.g. whether he is under some kind of obligation to act in a certain way). We will distinguish modality from the related concept of mood, which indicates the speaker's purpose in speaking.

In many languages, we find that a single affix actually encodes information from more than one of these domains, e.g. tense and aspect; or tense and modality. For this reason, many linguists prefer to treat Tense–Aspect–Modality (TAM) as a single complex category. In this chapter we treat them as being logically distinct, while recognizing that there is often some overlap in their grammatical expression.