因为…所以…: because… so…
复句 / complex sentences (因果)
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
因为 + cause，所以 + consequence
因为 introduces the cause, 所以 the result. As with 虽然…但是, Chinese likes to use both, while English picks one.
Usual order: cause first, result second. In speech one of them can be dropped: 因为下雨，我没去 / 下雨了，所以我没去.
To answer 为什么, start with 因为.
Examples
因为明天考试，所以我今天得复习。
Yīnwèi míngtiān kǎoshì, suǒyǐ wǒ jīntiān děi fùxí.Because I have an exam tomorrow, I have to revise today.
因为他很认真，所以成绩很好。
Yīnwèi tā hěn rènzhēn, suǒyǐ chéngjì hěn hǎo.He's very diligent, so his grades are good.
A：你为什么学中文？B：因为我对中国文化很感兴趣。
A: Nǐ wèi shénme xué Zhōngwén? B: Yīnwèi wǒ duì Zhōngguó wénhuà hěn gǎn xìngqù.A: Why are you learning Chinese? B: Because I'm very interested in Chinese culture.
昨天我生病了，所以没来上课。
Zuótiān wǒ shēngbìng le, suǒyǐ méi lái shàngkè.I was sick yesterday, so I didn't come to class.
Common mistakes
✗ 所以我没来，因为我生病了: if the cause comes second, drop 所以 → ✓ 我没来，因为我生病了.