The correlative pair pattern
Chinese loves pairs. English says "because it is raining, I am not going" with one conjunction. Chinese says 因为下雨,所以我不去 using both. Dropping 所以 is fine (因为下雨,我不去) but adding it is equally natural and often preferred in formal speech and writing. Dropping 因为 and keeping 所以 also works when context makes the cause obvious: 下雨了,所以我不去. Using both is the safest, clearest default.
Position: reason first, result second
The standard order is 因为 + cause, 所以 + result. Flipping is also allowed but changes emphasis: 我不去,因为下雨 puts the result first with the reason trailing, which is a more emphatic, emotional phrasing in speech. Formal written Chinese prefers the 因为...所以 order without flipping. If you flip, drop 所以 (you cannot have 所以 before 因为 in the same sentence).
因为他生病了,所以今天没来上课。
Yīnwèi tā shēngbìng le, suǒyǐ jīntiān méi lái shàngkè.
Because he is sick, he did not come to class today.
Alternatives and shortenings
由于 (yóuyú) replaces 因为 in formal or written contexts: 由于天气原因,航班取消 (due to weather, the flight is canceled). 因此 (yīncǐ) and 于是 (yúshì) are more formal equivalents of 所以, common in essays and news. In casual speech, Chinese speakers often drop both conjunctions and rely on context: 下雨了,我不去 (it is raining; I am not going). The causal link is inferred.
为什么 questions and 因为 answers
When answering a 为什么 (why) question, 因为 is the default opener: 为什么你不去? 因为我累了 (why are you not going? because I am tired). 所以 has no role in the answer because the question asks for the cause, not the result. In colloquial answers, 因为 can be dropped: 为什么你不去? 我累了 (why not? I am tired). The structure stays cause-first.
你为什么迟到了?因为路上堵车,所以晚了半个小时。
Nǐ wèishénme chídào le? Yīnwèi lùshang dǔchē, suǒyǐ wǎn le bàn ge xiǎoshí.
Why are you late? Because of traffic, I was delayed half an hour.