1. Both “因为”(because) and “由于”(because) can indicate the causal logic of the events. However, the casual logic of the sentence with “因(yīn)为(wéi)” is stronger. And when the casual logic of the sentence is from the result to the reason, we would use 因(yīn)为(wéi) rather than 由(yóu)于(yú).
(这zhè)家(jiā) 饭(fàn)店(diàn) 非(fēi)常(cháng) 受(shòu)欢(huān)迎(yíng)， 因(yīn)为(wéi) 这(zhè)里(lǐ)的(de)
This restaurant very popular because here
饺(jiǎo)子(zi) 特(tè)别(bié) 好(hào)吃(chī)
dumplings very delicious
(This restaurant is very popular, because the dumplings here are very delicious)
2. When answering the question of “why”, we should use 因(yīn)为(wéi) rather than 由(yóu)于(yú)
为(wèi)什(shén)么(me) 他(tā) 不(bù)高(gāo)兴(xìng)？ 因(yīn)为(wéi) 他(tā) 丢(diū)了(le) 他(tā)的(de) 手(shǒu)机(jī)。
Why he unhappy Because he lost his phone
(Why he is unhappy? Because he lost his phone)
Tips: If you are a Chinese learner at elementary or intermediate level, try to use "因为" when you are not sure.