因为 + Cause, 所以 + Result | 由于 + Cause, (因此/所以) + Result
因为...所以... (yīnwèi...suǒyǐ...) vs 由于 (yóuyú) — everyday cause vs formal/written cause
因为...所以... is the everyday, spoken way to link a cause and its result — 'because...so...': 因为下雨，所以我们没去公园 (because it rained, we didn't go to the park). In casual speech only one of the pair is often used (just 因为, or just 所以). 由于 (yóuyú) means the same 'because/due to', but is more formal and written — common in news, reports, and official notices — and it pairs more naturally with 因此 or 所以 for the result, and can also open a written sentence where 因为 would sound too conversational: 由于天气原因，航班延误了 (due to weather, the flight was delayed). Unlike 因为, 由于 cannot be used to answer a casual '为什么' question on its own. The deciding question: is this everyday conversation about a concrete, personal reason (→因为...所以...), or a more formal/impersonal explanation, often about a fact, policy, or written report (→由于)?
Examples
- 因为今天下雨，所以我们没去公园。yīn wèi jīn tiān xià yǔ, suǒ yǐ wǒ men méi qù gōng yuán
Because it rained today, we didn't go to the park.
- 由于天气原因，航班延误了三个小时。yóu yú tiān qì yuán yīn, háng bān yán wù le sān gè xiǎo shí
Due to weather conditions, the flight was delayed by three hours.
- 由于交通堵塞，他迟到了，因为这样，老板对他很不满意。yóu yú jiāo tōng dǔ sè, tā chí dào le, yīn wèi zhè yàng, lǎo bǎn duì tā hěn bù mǎn yì
Because of the traffic jam, he was late; because of that, his boss was quite unhappy with him.
Common mistake
✗ Wrong: 你今天为什么没来？由于我生病了。
✓ Right: 你今天为什么没来？因为我生病了。
由于 is too formal/written to answer a casual spoken 'why' question about yourself — everyday personal reasons use 因为, keeping 由于 for formal or written explanations.
Practice this pattern
Fill in the missing character:
＿为今天下雨，所以我们没去公园。
Because it rained today, we didn't go to the park.