We say 때문에 for the noun, and say -서 or -니까 for the verb stem. These three are most commonly being said. 감기 때문에 쉬고 있어요. I'm resting because of a cold 감기 걸려서 쉬고 있어요. I'm resting because I've got a cold. 감기 걸렸으니까요. Because I've got a cold. A little bit of information... -서 is a conjugational ending, it means it has 아/어 before itself, like -요 has 아/어. In 감기 걸려서, 걸리다 is the basic form of the verb, and the verb stem is 걸리-. Now you can figure out how 걸리 became 걸려서(걸리+어+서). :) And -서 cannot be conjugated with the past-tense verb. Well we don't say -므로 or -기 때문에 which are mostly used in written Korean. 왜냐하면 is used at times, but not often(Apologies to JIN). If you aren't in a formal situation, AiriLove, saying those formal words sounds pointed.

we use '왜냐하면'and '때문에" together in a sentence. when we explain a reason, we begin with 왜냐하면 and ends with 때문에. Ex) 나는 학교를 못갔다. 왜냐하면 아팠기 때문이다. i couldnt go to school. Because I was sick. 나는 아팠으므로 학교에 갈 수 없었다. i was sick, therfore i couldnt go to school.

Sadly, as I’m a native English speaker studying Korean in Korea. In English we basically have only one word that covers all “이유” sentences. I have found that the word “because” is one of the hardest things to say in Korean. It’s so complicated and there are so many rules and exceptions to each way of saying “because” examples are -왜냐하면 -때문에 -덕분에 -느라고 -는 바람에 -(으)니까 -아서/어서 -이므로 and the list of grammar ways of saying goes on and on. I have yet to have anyone including my Korean language teachers explain all of these well. It’s the little things like this that make me more and more sure that English really is an Easier language than Korean.