Whenever I speak in English, I tend to use the wrong grammar, but when I write, it's usually correct.
ご質問ありがとうございます。
・「Whenever I speak in English, I tend to use the wrong grammar, but when I write, it's usually correct.」
（意味)喋ると文法を間違ってしまうけど書くときは間違えない。
<例文>Whenever I speak in English, I tend to use the wrong grammar, but when I write, it's usually correct. I wonder why.
<訳>喋ると文法を間違ってしまうけど書くときは間違えない。 なぜでしょう。
参考になれば幸いです。
I think I have grammar worked out pretty well in my head, but then I start talking and make even the simplest of grammar mistakes.
この場合、次のような言い方ができますよ。
ーI think I have grammar worked out pretty well in my head, but then I start talking and make even the simplest of grammar mistakes.
「頭で文法は結構わかっていると思うけど、話し始めると簡単な文法も間違ってしまう」
to have grammar worked out in one's headで「頭で文法を理解する」ことを言えます。
ご参考まで！
アメリカ合衆国 
カナダ 
