to know the meaning of something that someone says

She explained the whole idea again, but I still didn't understand. 她把整个想法又解释了一遍，但我还是不明白。

Is there anyone here who understands Arabic? 这儿有人懂阿拉伯语吗？

I think he was calling from a club - it was so noisy, I couldn't understand a word he said. 我想他是在一个夜总会里打的电话——那头太吵了，他说的话我一个字也没听清。

to know why or how something happens or works

[ + question word ] We still don't fully understand how the brain works. 我们还未完全弄懂大脑是如何工作的。

to know how someone feels or why someone behaves in a particular way

My wife doesn't understand me. 我太太不理解我。

Sometimes I don't understand James. 有时候我搞不懂詹姆斯。

[ + question word ] You don't understand what it's like/how it feels to have to beg on the streets. 你不了解出于无奈才上街乞讨是什么样子／是什么样的感觉。

to think, especially wrongly, that someone means a particular thing

When he said three o'clock, I understood him to mean in the afternoon. 他说3点钟时，我误解为是下午。

When two people understand one another, they both know what the other means and wants and they have an agreement.

I wanted to make sure that we want the same things, that we really understand each other. 我想确保我们的目标一致并相互理解对方。

to know or realize something because you have been told it

[ + (that) ] I understand (that) you are interested in borrowing some money from us. 我听说你想从我们这里借些钱。

[ + that ] The Director had given her to understand (= told her) that she would be promoted. 主任跟她说她将会得到提拔。

[ + obj + to infinitive ] A secret buyer is understood to have paid $1 million for the three pictures (= there is unofficial news that this has happened). 据说，有一位不愿透露姓名的买家花了100万美元买走了那3幅画。

used when making certain that someone knows what you mean and will do as you want

I don't want you to see that boy again. Understand?/Do you understand?/Is that understood? 我不想让你再和那个男孩见面，明白吗？

used to refer to something such as a rule that everyone knows and accepts

In the library it is understood that people should talk quietly. 众所周知，图书馆里禁止大声喧哗。

make yourself understoodidiom C2 把自己的意思表达清楚