以下說話英文應該點講

嘗試了不同的譯法，希望幫到你：

When John is talking with you, please teach / remind him the right expression in English.
當 John 和你說話時, 請教導/提點他正確的英文說法

Please kindly teach / remind John the right expression in English when you are chatting with him.
懇請你教導/提點 John 正確的英文說法，當你和他閒談時。

When you are in a conversation with John, please grasp the opportunity and teach / remind him the right expression in English.
當你和 John 對話時, 請把握機會教導/提點他正確的英文說法

Please correct John's English when he is having a talk / chat with you.
請指正 John 的英文，當他跟你說話/閒談時。


註：
1. 可用 if 代替 when。兩者的分別在於可能性，when 暗示了 「John 一定會和你對話」，if 則表示「John 或者會和你對話」，看你的情況決定用哪個吧。
2. 如果你想說「請教導/提點他某東西的正確英文說法」，那麼可用 the correct way of saying something in English 來代替 the right expression in English。

樓上的 correct way in English speaking 其實很容易讓人誤解為「說英文的正確方法，如咬字、發音等」，而非「正確英文說法」。

When John is having conservation with you, please teach/remind him the correct way in English speaking.

When John said to him, please exhibitions?