Reason Clauses: As, Since & Because
Level B1 · Conjunctions & Linking Words
as, since, and because all introduce a reason for something, but they differ slightly in emphasis and position. because gives the main, most important reason and usually answers the direct question 'why?'; as and since introduce a reason the listener probably already knows or that's less central to the sentence, often placed at the beginning for a softer, more explanatory tone. I couldn't come because I was ill directly answers 'why didn't you come?'; As/Since I was already in town, I decided to visit her assumes the listener already knows you were in town, and treats that as background leading to the main point.
How each reason word is used:
- because + clause gives the direct, central reason, and can appear at the start or end of the sentence, though it's more common at the end, directly answering 'why': She left early because she was tired; Because she was tired, she left early.
- as and since + clause introduce a reason that's often already known or assumed, and are more common at the beginning of the sentence, setting up background before the main point: As it was raining, we stayed inside; Since you're here, let's start the meeting.
- as and since are largely interchangeable in this reason sense, though since can also mean 'from that time', so context decides which meaning is intended: Since I moved here (reason: because I moved here / time: from when I moved here) — because never has this time meaning.
- because of + noun/-ing (not a full clause) is used when there's no separate subject and verb, just a noun phrase causing something: The match was cancelled because of the rain (not because of it rained).
The most common mistake is using because of before a full clause with its own subject and verb — because of it was raining is wrong; because of needs a noun (because of the rain), while because needs a full clause (because it was raining). A second mistake is confusing the reason since with the time since, which can create ambiguity — Since she left, I've been busy could mean either 'because she left' (reason) or 'from the time she left' (time); when the sentence could be misread, choose because instead to make the reason meaning unambiguous.
Look for as, since, or because introducing a clause that answers 'why?' — because usually signals the main, direct reason and sits comfortably at the end of the sentence; as/since often open the sentence with a reason treated as already known or less central. Check whether since could also mean 'starting from a point in time' — if the sentence could be misread that way, prefer because for a reason.