Is there a word for someone who uses improper grammar?

There isn't one specific word for someone who uses improper grammar. However, here are some terms you could use depending on the context:

Informal:

* Grammatically challenged: This is a humorous and lighthearted way to describe someone who makes grammatical errors.

* Grammar Nazi: This term is used sarcastically or ironically for someone who is overly critical of grammatical mistakes.

* Ungrammatical: This is a more neutral term, simply stating that someone's grammar is not correct.

Formal:

* Someone who uses incorrect grammar: This is the most accurate and direct way to describe the situation.

* Someone whose grammar needs improvement: This is a more polite and constructive way to phrase it.

It's important to note that the term "grammatically challenged" can be considered offensive by some people, as it implies that someone is lacking in intelligence or ability. It's best to be mindful of your audience and choose your words carefully.