something is obligeation that you don' t do it

you are obliged to do it or is a necessity

Not necessity

must = ......

I / WE questions suggestions or offers

Predictions, Warnings, offers, promises, threats, requests, suggestions, on-the-spot decisions, opinions, hopes and fears

polite or formal requests

POLITE OR FORMAL OFERS AND SUGGESTIONS

that is a good idea or not

lack of necessity

avblity or not

can in past + particular situation

do something with effor