Cause and Effect: Because, Since, As

Because, since, and as all introduce a reason — they explain why something happened. "I stayed home because I was ill" gives the reason for staying home. All three are conjunctions followed by a clause (subject + verb). The main difference is emphasis: because puts strong focus on the reason and usually comes after the main clause. Since and as introduce reasons that are already known or less important, and often come at the start of the sentence.

Compare these examples: "I missed the bus because I woke up late" (the reason is the main point) vs "Since you already know the answer, I won't explain again" (the reason is background information). As works like "since" for known reasons: "As it was getting dark, we decided to go home." Be careful: "since" can also mean "from that time" and "as" can also mean "while" — context makes the meaning clear. When answering "why" questions, always use "because," not "since" or "as": "Why were you late?" — "Because I missed the bus." This distinction is important for B1 level learners.