因为他忘了她的生日，所以她那么生气。

Yīnwèi tā wàng le tā de shēngrì, suǒyǐ tā nàme shēngqì.

Because he forgot her birthday, she's that angry.

因为他在美国读书，所以英文这么好。

Yīnwèi tā zài Měiguó dúshū, suǒyǐ Yīngwén zhème hǎo.

Because he is studying in America, so his English is this good.

因为我生病了，所以不能上班。

Yīnwèi wǒ shēngbìng le, suǒyǐ bù néng shàngbān.

Because I'm sick, I can't go to work.