老师不知道他为什么没来上课。

Lǎoshī bù zhīdào tā wèishénme méi lái shàngkè.

The teacher doesn’t know why he didn’t come to class.

老师早就知道这件事了。

Lǎoshī zǎo jiù zhīdào zhè jiàn shì le.

The teacher already knew about this matter long ago.