If something makes you sad when it ends.
It must have been pretty wonderful when it was happening.

From: William Hill 《This is US》《我们这一天》

如果一件事结束时让你难过，
那它发生的时候一定很美好。

1. make sb/sth/yourself + adj 使变得，使成为
The news made him very happ
这则消息使他非常高兴。

Can you make yourself understood in English.
你能用英语表达你的意思吗？

2. must have been ... 一定是。。。(对过去实事的推测)
Your trip must have been fun.
你这次旅行一定很开心吧。

There must have been a pretty strong selective pressure.
一定有很强的选择压力。

It must have been very expensive moving such big trees.
移动这么大的树一定很费钱。

It must have been frightening, meeting strange men.
你和陌生人见面，心里肯定很害怕吧。

You must have done something right.
你一定做对了什么事。