適切な文法に則った会話ができないと表現したいなら、いくつかの例があります。
I can't use... / I don't use...
...the correct grammar.
...proper grammar.
...grammar correctly.
...grammar properly.
I need to work on/improve...
...my grammar usage.
...my usage of grammar.
教師にこれをつたえられませす:
Can you help me with my grammar usage? I can't speak English properly because I make a lot of grammatical errors.