不得不

have no choice or option but to / cannot but / have to / can't help it / can't avoid

no; not so / (bound form) not; un-

must not / may not / not to be allowed / cannot

to obtain / to get / to gain / to catch (a disease) / proper / suitable / proud / contented / to allow / to permit / ready / finished

structural particle: used after a verb (or adjective as main verb), linking it to following phrase indicating effect, degree, possibility etc

to have to / must / ought to / to need to