The paper puts forward the concept of act structure, assuming that both syntactic and speech act have their own structure. They are independent of each other in the structure, yet relevant to each other in some way. A direct correlation forms as Syntactic predicate marks the speech act in the form, otherwise, an indirect correlation, respectively resulting in a directly-related sentence and indirectly-related sentence. Speech acts are acts of interpersonal interaction; the interpersonal relations established by the speech are embodied in the basic structure of speech acts. Interpersonal behavior carries on modality and whether the speech act is involved in the predicate marks the boundary between tense and modality. So, a directly-related sentence belongs to the modal category, while an indirectly-related sentence the tense category. Performative, imperative, modal and virtual sentences are directly-related with their predicates directly marking their speech acts; narrative sentences are indirectly-related without their predicates marking the speech act in the form at all.