frog in a well 井底之蛙

“井底之蛙”，井底下的青蛙（frog in a well），只能看到井口那么大的一块天。比喻见识短浅的人。可以翻译为“person with a narrow view，person with a very limited outlook”等。

例句：

如果他们纵容自己呆在校园提供的安全感里，他们就会变成井底之蛙，永远无法抓住适应社会的机会。

But if they indulge in this sense of security the campus provides for them, they will become a frog in a well, never catching the opportunity to adapt to society.