to be certain that something exists

Do you believe in ghosts? 你相信有鬼吗？

to be confident that something is effective and right

They don't believe in living together before marriage. 他们认为婚前同居不好。

He believes in saying what he thinks. 他认为说出自己的想法是对的。

to trust someone because you think that they can do something well or that they are a good person

Gradually, since her divorce, she's beginning to believe in herself again. 自从离婚后，她渐渐开始对自己有信心了。