how can i use the word "that" as the word "que"?
I think that, if you don't come straight away, he won't be pleased.
J'ai déjà lu le livre que Pierre m'a donné.
I've already read the book that Pierre gave me.
In other words, that is a conjunction (as in the first sentence) but also a relative pronoun (as in the second sentence).
Of course, that has other grammatical functions but you don't translate these by que.