“As”, “since” and “because” can be used at the beginning, in the middle and at the end of a sentence. “As” and “since” are more formal than because, and they focus more on the RESULT. “Because” focuses on the REASON. As/Since: As/Since Tom wants to buy a new house, he will work harder. Beginning, result (buying a new house). Tom, as/since he wants to buy a new house, will work harder. Middle, result. This one is normally used in formal contexts only. Tom will work harder as/since he wants to buy a new house. End, result as well. Because: Because Tom wants to buy a new house, he will work harder. Beginning, reason (the focus is on “he will work harder”). Tom, because he wants to buy a new house, will work harder. Middle, reason. This one is normally used in formal contexts only. Tom will work harder because he wants to buy a new house. End, reason as well. As you can see, this is not really a matter of whether the reason is already well known or not; this is about the level of formality one wants to use and what one wants to focus on (result or reason). I hope that helps.

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