obligation/duty/necessity. must:express duty/strong obligation to do sth, shows that sth is essential. we generally use must when the speaker has decided that sth is necessary (i.e. subjective.
obligation/duty/necessity. must:express duty/strong obligation to do sth, shows that sth is essential. we generally use must when the speaker has decided that sth is necessary (i.e. subjective.Have to: express strong necessity/obligation.