The indefinite article is a basic part of English grammar.

Every Saturday, I practice soft tennis in the park.

His clumsy actions caused an accident.

His clumsiness caused the project to be delayed.

That word is improper for this situation.

I think that comment is improper.

That outfit is unsuitable for this occasion.

I think that behavior is completely inappropriate.

We are planning to go rafting this weekend.

He jumped into the fire with intrepidity.