You don't know what it is.

你不知道那是什么。

If you don't know the name you would end the letter ‘Yours faithfully'.

如果你不知道名字，你会以“忠实的”结束信件。

You don't know what it means and ask the shop assistant to explain.

你不知道这是什么意思，于是请店员解释。

The highest form of ignorance is when you reject something you don't know anything about.

无知的最高形式是拒绝你一无所知的东西。

If you don't know the answer, just leave a blank.

如果你不知道答案，就留空。

It's easy to generalise when you don't know the specifics.

当你不知道具体情况时，很容易进行概括。

Avoid making hasty judgments about people you don't know well.

避免对你不太了解的人做出匆忙的判断。

If you don't know the answer, you can look up the book.

如果你不知道答案，可以查阅书本。

If you don't know how to swim, you might drown in the river.

如果你不会游泳，你可能会在河里溺水。

When you don't know what to say, remember silence is gold.

当你不知道该说什么时，记住沉默是金。