We can also use it to remind ourselves to do something. They express strong obligation, the opinion of the speaker and are associated with a more formal, written style. MUST and MUST NOT are used in formal rules and regulations and in warnings. Obligation and Advice: MUST / HAVE TO / SHOULD / OUGHT TO / HAD BETTER
OUGHT TO, as well as SHOULD, + HAVE + PAST PARTICIPLE are used to talk about an expectation that something happened, has happened, or will happen.
They indicate an assumption based on our knowledge of people and things, their routine, character and qualities. WILL and WON'T are also used to express what we believe or guess to be true about the present.
All of them (except for NEED) can express degress of certainty, probability or possibility of an event.