Formal:
* It is improper to wear casual clothing to a formal dinner.
* The company's financial statements contained improper accounting practices.
* Her use of language was considered improper in that setting.
Informal:
* It's improper to interrupt someone while they're speaking.
* That's an improper use of that word.
* He was fired for improper conduct.
Figurative:
* Her improper behavior at the party caused a major scene.
* The improper functioning of the engine led to a breakdown.
Remember, the specific usage of "improper" will depend on the context of your sentence.