How did he meet your mother?—I don't know.他是怎么见到你妈妈的？——我也不确定。

I don't know if … or I don't know whether …我不确定是否…

I don't know whether she's coming.我不确定她是否要来。

I don't know how … or I don't know what … or I don't know why …(indicating criticism)真想不到, 居然(表示批评某人的行为)

I don't know how he could do that to his own daughter.真想不到他竟对自己的女儿这样。

I don't know how you can think that!我想不到你会那样想！

I don't know that …我不认为…

I don't know that it's made the decision any easier.我认为这不会使作出决定变得容易。

I don't know that I agree.我并不同意。

It's really expensive.—Oh, I don't know. It'll last for ages.这确实是太贵了。——哦, 我可不这么认为。它能用很久。