because, as, since or for

Word	Explanation	Example
because	The reason is very important in the sentence, so because comes in the sencond clause of the sentence.	I read this book because I like it.
as	The reason is already known.	As we read, we learn.
since	The reason is already known. (more formal than as)	Since we were in the computer lab, our English has improved.
for	The reason is given at the end of the sentence.	We went to a small restaurant - for we were hungry.

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