Let's differentiate the clauses: BECAUSE -more common than as or since -used when the reason is the most important part of the sentence -you CAN put it in the beginning of sentences specially if you want to put extra focus to the reason example: Because I've known him since we were little, I got surprised to see him grow into such a fine young man. -can be used on its own without the main clause in speaking or informal writing: A: Would you like to go to school there? B: Yes. A: Why? B: Because my best friend goes there. (I would like to go to school there because my best friend goes there.) however, you cannot do this in formal writing: In 1998, the government introduced a new import tax because people were importing cars from abroad. NOT: … a new import tax. Because people were importing cars from abroad. -because is also used in questions where the speaker proposes a reason: Are you feeling unwell BECAUSE you ate too much? NOT: Are you feeling unwell since you ate too much? or … as you ate too much? --------------------------------------------------- AS and SINCE -used when we want to focus more on the result than the reason, as you may know, as and since are used when the reason is already well-known, thus, making the reason usually LESS important *result* <reason> *I hope they’ve decided to come* as *They’re rather expensive* since, <they’re quite hard to find.> . -more formal than because -often used at the beginning of sentences hope this helps as well!:) ---------------------------------------------- https://dictionary.cambridge.org/us/grammar/british-grammar/as-because-or-since