modality, mood, mode - verb inflections that express how the action or state is conceived by the speaker

obligation, responsibility, duty - the social force that binds you to your obligations and the courses of action demanded by that force; "we must instill a sense of duty in our children"; "every right implies a responsibility; every opportunity, an obligation; every possession, a duty"- John D.Rockefeller Jr

assertive - inclined to bold and confident assertion; aggressively self-assured; "an energetic assertive boy who was always ready to argue"; "pointing directly at a listener is an assertive act"

grammar - studies of the formation of basic linguistic units