why do we use past tense when we talk about our dream?急!

We had the dream when we slept. But now we have waken up. That means our dream was something happened in the past. Therefore, we should use past tense when we talk about our dream.

我們在睡覺時做夢，但現在我們已睡醒了，所以夢境是過去發生的事情，所以要用過去式。

you use past tense because.....

you can talk about your dreams ONLY because you already had the dreams. eg last night

you can't possibly foresee your dreams. eg for tonight

You must use past tense to describe what happened in the past,
this includes, when you went to sleep, and whatever that happened while you were asleep, ie the dreams you had.