'bad' is an adjective, 'badly' is an adverb [many adverbs are {adjective}+ly]. His French is bad [adjective modifying the noun 'French']. He speaks French pretty badly [adverb modifying the verb 'speaks'] The movie was pretty bad [adjective modifying the noun 'movie']. The movie we saw was pretty bad [still need to modify the noun 'movie' | 'we saw' is just extra information and not needed in the sentence].

Badly modifies a verb, so he speaks badly. Bad modifies a noun, so the movie was pretty bad.