我不明白用英文怎麼表達？

I don't understand the instructions.

我不懂這些指令的意思。

I don't understand what you are talking about

我不明白你在説什麼。

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

“我不明白，”巡官一邊嘟囔道，一邊用手捋了一下頭髮。

I don't understand how the two facts relate.

我不明白這兩個事實有什麼聯繫。

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

我不懂你在説什麼。