pretty vs passably

pretty is an adjective but passably is not an adjective.

Word	Noun	Adjective	Verb	Adverb
pretty	No	Yes	No	Yes
passably	No	No	No	Yes

As adverbs, pretty and passably are synonyms defined as:

Other synonyms of pretty include fairly, jolly, middling, moderately, reasonably, somewhat.

pretty (adjective)	passably (adjective)
(used ironically) unexpectedly bad
pleasing by delicacy or grace; not imposing

pretty (adverb)	passably (adverb)
to certain extent or degree	to certain extent or degree