Antonyms of adj ungrammatical

1 sense of ungrammatical

Sense 1
ungrammatical (vs. grammatical), ill-formed -- (not grammatical; not conforming to the rules of grammar or accepted usage)

grammatical (vs. ungrammatical), well-formed -- (conforming to the rules of grammar or usage accepted by native speakers; "spoke in grammatical sentences")

Similarity of adj ungrammatical

1 sense of ungrammatical

Sense 1
ungrammatical (vs. grammatical), ill-formed -- (not grammatical; not conforming to the rules of grammar or accepted usage)
       => incorrect -- ((of a word or expression) not agreeing with grammatical principles)