when I was passing by his room,he was talking with __ teacher.填the other?another?why?

another 因为the other特指其他人里的具体一个人，而another是泛指其他任何人抑或一群人。

ANOTHER

when I was passing by his room, he was talking with _another_ teacher.
因为“I”是“was passing by” 跟那个he was talking with __ teacher的那个teacher肯定不是同一个人。所以是another teacher了。如果是强调只要两个人，就会有one(一方)...the other(对方).
祝你学习进步！

another.

another

another, 表示的是泛指"另一",
the other, 表示的是 二者中的 "另一",

因为这里没有铺垫,说room里只有两个teacher, 所以不用the other.

要根据上下文.
前文如果出现了两个teacher,那就填the other
前文如果没有出现,或者只有一个teacher,那就填 another
如果这是个单句,没有上下文,填another

the other 一般是 有两个人时，说另一个人，就用 one..... the other
another adj.又一个；再一个；另一的；其他一种pron.另一个，别个；再一个
Try to give it another place.
设法给它另外位置。
所以 选 another