For the clause as exchange, dialogue consists of four fundamental functions: statement, question, command and offer. The difference between them lies in the relationship of the Subject to the Finite. The Finite is the element that indicates either tense (is/was, do/did) or modality (can/must) and is often “fused into a single word” (H & M, p.111). He plays tennis, for example, can be expanded to He does play tennis. The Subject and the Finite can be identified through the Mood tag: He plays tennis, does he?
For statements, the relationship is Subject + Finite. This is called the declarative Mood:

I play tennis
Subject Finite Residue
For questions, the relationship is reversed. This is called the interrogative Mood. The interrogative Mood may either indicate polarity or content:

Do you play tennis?
What sports do you play?
Residue Finite Subject Residue
For commands, however, both the Subject and the Finite may be omitted. This is called the imperative Mood:

Play tennis!
We may, however, mark the imperative Mood to make it more emphatic or inclusive:

Do play tennis!
Let’s play tennis!
Subject Finite Residue

Offers, however, have no particular realization, although they often employ modal verbs.