Improper and Ill-formed

Improper adjective - Not appropriate for a particular occasion or situation.

Ill-formed and improper are semantically related In some cases you can use "Ill-formed" instead an adjective "Improper".

Nearby Word: improperly

Ill-formed adjective - Not grammatical; not conforming to the rules of grammar or accepted usage.

Improper and ill-formed are semantically related in not using the correct rules of grammar topic. You can use "Improper" instead an adjective phrase "Ill-formed".