我的袋子不见了，怎么办?

Wǒde dàizi bùjiàn le, zěnme bàn?

I can't find my bag, what should I do?

你怎么会忘记我的生日?

Nǐ zěnme huì wàngjì wǒde shēngrì?

How could you forget my birthday?

我怎么不知道她有男朋友？

Wǒ zěnme bù zhīdào tā yǒu nánpéngyou?

How did I not know she has a boyfriend?