as, because, or since

As, because, and since are three conjunctions that introduce subordinate clauses connecting a result and a reason:

• She decided to go to the conference in Seoul, as he was in Korea anyway.
• Are you mad at me because I opened the letter?
• Sujin teaches her students on Google Meet, since she lives in Spain.

Because

We use because when we want to focus more on the reason. More commonly, the because clause emphasizing the reason ends the sentence. We use a comma after the because-clause when we put the because-clause at the beginning of a sentence:

• She got the promotion over four other candidates because she knew the system best.
• Because breathing is something we do automatically, we rarely think about it.

As and since

We often use as and since when we want to focus more on the result than the reason. We usually put a comma before since after the main clause:

• They’ve decided to come as they had nothing to do.
• They’re quite expensive, since they’re hard to find.
• Since we can learn everything from home with computers and telephones, we need not dress up for school any more.
• As everyone already knows each other, we’ll get straight into the topic.

If you ask a question and you also put up the explanation or cause, in that question, we use because and not since:

• Are you okay because you were late?