也不是绝对不可以填其他的.根据这句意思最好填before,因为but表示转折.when也行,但不够before好.
He made a mistake,but he corrected it (before) the situation got worse.
他犯了一个错误,但在情况转坏之前他改正了.
He made a mistake,but he didn't correct it (until) the situation got worse.
他犯了一个错误,但直到情况转坏了他才改正.
He made a mistake,but he corrected it (when) the situation got worse.
他犯了一个错误,但在情况转坏的时候他改正了.