In Korean, “because” clauses can be expressed with ~기 때문에 :

돈이 없기 때문에 대학교를 못 다녔어요 = I couldn’t attend college because I don’t have money

(돈 = money. 이 = particle. 없기 때문에 = because I don’t have. 대학교 = college. 를 = particle. 못 다녔어요 = couldn’t attend.)

The clause “because I don’t have money” is placed at the beginning of the sentence in Korean (돈이 없기 때문에) and then the rest of the sentence which is, “I couldn’t attend college” (대학교를 못 다녔어요) is placed behind the “because” clause.

Compare the sentence above with the sentence below :

돈이 없었기 때문에 대학교를 못 다녔어요 = I couldn’t attend college because I didn’t have the money

(돈 = money. 이 = particle. 없었기 때문에 = because I didn’t have. 대학교 = college. 를 = particle. 못 다녔어요 = couldn’t attend)

With the second sentence, the first clause in Korean is 없엇기 때문에, but the first clause in the first sentence is 없기 때문에. The second sentence has the first clause in the past tense, so it’s going to be “didn’t have” as opposed to the first sentence, which means “don’t have”.

서울까지 운전하기 싫어요. 왜냐하면 교통이 항상 심해요 = I don’t like to drive to Seoul. Because the traffic is always heavy.

(서울 = Seoul. 까지 = to. 운전하기 싫어요 = don’t like to drive. 왜냐하면 = because. 교통 = traffic. 이 = particle. 항상 = always. 심해요 = heavy/severe)

In the sentence above, the reason or the “because” is the secondary clause.

Now let’s compare this with :

서울까지 운전하기 싫어. 왜냐면 교통이 항상 심해 = I don’t like to drive to Seoul. Because the traffic is always heavy.

(서울 = Seoul. 까지 = to. 운전하기 싫어 = don’t like to drive. 왜냐면 = because. 교통 = traffic. 이 = particle. 항상 = always. 심해 = heavy/severe)

In the second sentence 왜냐면 was used. In the first sentence, 왜냐하면 was used. Both contain the same meaning, but 왜냐면 is more colloquial and used in spoken speech.