一、表示：变成，成为 (=start to be)，后接名词、形容词或过去分词。

They became great friends. 

他们成了很好的朋友。

He became famous when he was only a child. 

他在小时候就出名了。

They had just become engaged. 

他们刚刚订婚。

二、become后一般不接不定式 (此时可用 come / begin＋不定式)。

如：他开始对英语感兴趣了。

He began / came to like English. 

He became interested in English.

三、become也可表示天气、社会的发展变化。

Our country is becoming stronger and stronger.

我们的国家日益强大。

It's becoming colder and colder.

天变得越来越冷。