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He didn't know he had already made a big mistake

朋友
He didn't know he has already made a big mistake當然是錯，因為made a big mistake是”He didn't know”之前所發生的事，所以，要用past perfect tense
所以，照理（像各位的意見一樣），
He didn't know he had already made a big mistake（是對的）
單單看這句就對；但若連同上下文看，可能要轉用其他時式。所以，若是文章，請你交代前文後理；若是練習，也請你代交練習屬於哪個topic，並交代全題目。
若都沒有問題，就請你問老師你錯在何處，這是最直接的方法。若你得到老師的答案後，也請你向大家交代，待我們看看，他的解釋正確與否。（雖知香港英文老師真是參差不齊）。
你補充資料後，我們會再為你作答。

Maybe you can try this:

He didn't know that he had already made a big mistake.

He didn't know he had already made a big mistake

2008-01-16 14:55:10 補充：
THIS IS NOT COPY BY hckherbert

He didn't know he has already made a big mistake 有咩wrong 呀???
無錯, 但轉一個 format 似乎更英語化:
He just didnt know there is a big mistake already made.

He didn't know he had made a big mistake already!