ungrammatical vs ill-formed

ungrammatical and ill-formed both are adjectives.

Word	Noun	Adjective	Verb	Adverb
ungrammatical	No	Yes	No	No
ill-formed	No	Yes	No	No

As adjectives, ungrammatical and ill-formed are synonyms defined as:

ungrammatical and ill-formed: not grammatical; not conforming to the rules of grammar or accepted usage

ungrammatical (adjective)	ill-formed (adjective)
not grammatical; not conforming to the rules of grammar or accepted usage	not grammatical; not conforming to the rules of grammar or accepted usage