我不明白用英文怎么表达？

I don't understand the instructions.

我不懂这些指令的意思。

I don't understand what you are talking about

我不明白你在说什么。

'I don't understand,' the Inspector mumbled, passing a hand through his hair

“我不明白，”巡官一边嘟囔道，一边用手捋了一下头发。

I don't understand how the two facts relate.

我不明白这两个事实有什么联系。

I don't understand what you're talking about.

我不懂你在说什么。