因为 (yīnwèi) means "because" and introduces the reason. 所以 (suǒyǐ) means "so" and introduces the result. In Chinese the two are very often used together in one sentence, with the reason first and the result second.
因为 + reason，所以 + result.
In English you say either "because" or "so", never both in one sentence. In Chinese, using 因为 and 所以 together is completely normal and very clear. The reason comes first, then a comma, then the result.
You don't always need both words. In everyday speech people often use just 所以, with the reason first and no word in front of it, or just 因为.
A 因为 clause can also come after the result, like "because" in English. In that order, don't add 所以.
因为 can go before or after the subject: 因为我太忙 and 我因为太忙 are both correct. 所以 is different: it always starts its clause, in front of the subject. Say 所以我没有去, never 我所以没有去.
If both parts of the sentence are about the same person, you only need to say the subject once. Negatives with 不 and 没有 stay inside their own clause, just as in a normal sentence.
The most common mistake is to mix up the reason and the result: putting 所以 in front of the reason, or 因为 in front of the result. Before you choose, ask yourself which part explains why. That part gets 因为. The part that happens as a consequence gets 所以.
The two sentences below say the same thing in two orders. Notice that 因为 always stays with the reason, 路很远.
To ask "why", use 为什么 (wèishénme), usually after the subject and before the verb. Answer with a 因为 clause on its own; this is a normal short answer, just like "Because …" in English. Don't start the answer with 所以.
Practise this grammar point on paper: 19 exercises using only HSK 2 words, with an answer key on a separate page. Free to print and copy for your class.
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