Cause and effect (formal): 由于……因此
复句 / complex sentences
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
由于 (yóuyú) = "due to, owing to"; 因此 (yīncǐ) = "therefore, as a result". This is the more written / formal version of 因为……所以, very common in news and texts.
由于 can pair with 所以 or 因此, but in careful style avoid 因为 + 因此.
由于 can also take just a noun: 由于天气原因，航班取消了 (due to the weather, the flight was cancelled).
Examples
由于空气污染严重，很多人戴上了口罩。
Yóuyú kōngqì wūrǎn yánzhòng, hěn duō rén dàishang le kǒuzhào.Due to severe air pollution, many people put on face masks.
这条河以前很脏，因此政府决定保护它。
Zhè tiáo hé yǐqián hěn zāng, yīncǐ zhèngfǔ juédìng bǎohù tā.This river used to be dirty, so the government decided to protect it.
由于大家的努力，城市变得更干净了。
Yóuyú dàjiā de nǔlì, chéngshì biàn de gèng gānjìng le.Thanks to everyone's efforts, the city has become cleaner.
Common mistakes
✗ 因此下雨，我没去。 → 因此 introduces the result, not the cause. ✓ 由于下雨，因此我没去。
因此 opens the second clause; it never goes at the end.