English students are taught that connectives (joining words) such as "because" and "if" are used in complex sentences to show that one clause depends on another, e.g. "He missed the bus because he overslept".
In the sentence above, the basic piece of information conveyed is that "He missed the bus". Connectives are helpful because they allow us to make logical connections between different pieces of information. So here, there is a reason why he missed the bus, which is that "he overslept". Rather than writing two unconnected sentences, "He missed the bus. He overslept", using the word "because" elegantly expresses the reason the two pieces of information are related. Clever, eh?
It's important to choose the correct connective, however. If we replace "because" with "so", the sentence no longer works: "He missed the bus so he overslept". "So" would work in another sentence: "He missed the bus so he walked instead". Think carefully about the logical link you are trying to express through the use of a connective.
See if you can get full marks in this quiz about connectives.