Synonyms:
* Formal: Bound, compelled, required, forced, indebted, beholden
* Informal: Have to, must, need to, got to, should
Antonyms:
* Formal: Free, exempt, unconstrained, unobligated, independent
* Informal: Don't have to, can choose not to, optional
Note: The specific synonym or antonym that's most appropriate depends on the context of the sentence.