What exactly “reason” means here
Because, since, and as introduce a subordinate clause of reason: a complete clause with its own subject and predicate that explains why the action in the main clause happens. This is a discourse connection expressing “why,” not “therefore”—result connectors (for example, therefore) and prepositional constructions (because of), which work differently, are not included here.
All three words answer the hidden question “why?”, but they differ in how new the reason is to the listener and in where the subordinate clause naturally appears in the sentence.
Because the delivery driver got stuck in traffic, the parcel arrived two hours late.
Because the delivery driver got stuck in traffic, the parcel arrived two hours late.
Because introduces a new reason; here the subordinate clause comes first, so a comma is needed before the main clause.
Since the shop was already closed, we ordered dinner online instead.
Since the shop was already closed, we ordered dinner online instead.
Since comes at the beginning of the sentence; the reason (the shop was closed) is presented as an obvious circumstance, not as new information.
As the printer had run out of paper, Sofia finished the copies at the library instead.
As the printer had run out of paper, Sofia finished the copies at the library instead.
As introduces a reason here, not time or manner—check the meaning, not just the word.
Form of a subordinate clause of reason
After because/since/as, a complete clause with its own subject and predicate always follows—not a bare noun and not an infinitive. This is the main difference from because of/due to, which require a noun phrase.
When the subordinate clause of reason comes before the main clause, a comma is placed between them. When the main clause comes first and the reason follows, a comma is usually not needed.
Because / Since / As + subject + predicate(+...), main clause. Or: main clause + because / since / as + subject + predicate(+...).
- subject / S —
- The grammatical subject: who or what the clause is about; the verb agrees with it. In The book was written, the book is the subject, although someone else does the action.
- clause —
- A construction with a verb, often with its own subject: when I arrived. A non-finite clause may leave the subject unspoken: to get home.
Since the elevator was out of service, residents used the stairs for two days.
Since the elevator was out of service, residents used the stairs for two days.
The subordinate clause (since...) comes first, so a comma is needed after it.
Residents used the stairs for two days because the elevator was out of service.
Residents used the stairs for two days because the elevator was out of service.
The meaning is the same, but the main clause comes first, so no comma is needed before because.
Because—the most direct way to name a reason
Because is the most neutral and common way to name a reason. It naturally appears both at the beginning and at the end of a sentence, and it gives no indication of whether the listener already knows the reason.
Because is the standard way to answer a Why...? question directly. Since and as sound unnatural, almost incomplete, in a short answer to “why?” because they more often introduce a reason as part of a longer statement.
Why did you call the landlord? — Because the heating stopped working again.
Why did you call the landlord? — Because the heating stopped working again.
Because is a natural direct answer to Why...?; Since/As would sound cut off here.
Because the contract hadn't been signed, the delivery was put on hold.
Because the contract hadn't been signed, the delivery was put on hold.
Because is equally natural at the beginning of a sentence—not only at the end.
Since / As—when the reason is already known
Since and as often introduce a reason that the listener already knows, considers obvious, or that has just been discussed. That is why they especially often appear at the beginning of a sentence—as if the writer were saying, “you already know this, so...”.
As is more common in written and formal styles; since is natural in both conversation and correspondence. Neither commonly begins a short, self-contained answer to “why?”.
Since we've already discussed the budget, let's move to the next item on the agenda.
Since we've already discussed the budget, let's move to the next item on the agenda.
The reason (the budget has already been discussed) is known to both speakers, so since sounds natural.
As most of the team had already left for the day, the manager rescheduled the call for the morning.
As most of the team had already left for the day, the manager rescheduled the call for the morning.
The tone is formal and written; as introduces a reason known from the context of the workday.
A trap: since and as have other meanings
Since can indicate a starting point in time: I haven’t spoken to her since Monday. A perfect tense in the main clause often helps identify the temporal meaning, but it is not the only possible form: It is two years since we met. Verb tense alone does not resolve every ambiguity: the causal Since we have finished, we can leave also contains a perfect tense.
As can also mean “while/at the same time” (time) or “in the same way as” (manner). The same sentence with as can sometimes be read either as expressing a reason or as meaning “while,” so in clear teaching examples, choose a context that points definitely to only one of the two meanings.
I haven't spoken to my old classmates since we graduated.
I haven't spoken to my old classmates since we graduated.
Since means TIME here (from graduation until now), not reason; the clue is Present Perfect + an event that provides a starting point.
As the guests were arriving, the host was still setting the table.
As the guests were arriving, the host was still setting the table.
In this context, as describes the simultaneous arrival of the guests and the setting of the table. In another context, as can introduce a reason; rely on the meaning of the whole message.
As the results were still inconclusive, the researchers repeated the experiment.
As the results were still inconclusive, the researchers repeated the experiment.
Here, in contrast, as expresses a reason: the inconclusive results logically explain why the researchers repeated the experiment.
The boundary with Because of / Due to
Because of and due to introduce a noun phrase, not a complete clause: because of the delay, due to heavy traffic. Because, since, and as cannot do this—and conversely, because of/due to cannot come before a clause with its own predicate.
The most common mistake at this level is putting because of before a complete clause (because of the traffic was heavy). This rule practices only the clause construction; because of / due to itself is discussed in detail in the neighboring rule in this topic.
Because the road was closed for repairs, drivers used the ring road instead.
Because the road was closed for repairs, drivers used the ring road instead.
The clause (the road was closed for repairs) requires because, not because of.
Because of the road closure, drivers used the ring road instead.
Because of the road closure, drivers used the ring road instead.
The noun phrase (the road closure) requires because of, not because.
Negation and questions in a clause of reason
Negation in a subordinate clause of reason is formed in the same way as in an ordinary sentence: an auxiliary verb is needed (isn't, didn't, hasn't, wasn't, and similar forms), not the bare particle not before the main verb.
Because, since, and as do not themselves introduce questions about reasons: to ask, use Why...?, while because remains the natural way to answer. Since/as sound unnatural when used as an answer to a direct question.
Since the invoice hasn't arrived yet, we can't process the payment today.
Since the invoice hasn't arrived yet, we can't process the payment today.
Hasn't arrived—auxiliary has + not + V3; the bare form not arrived yet would be incorrect.
Why did the meeting start late? — Because the previous call overran.
Why did the meeting start late? — Because the previous call overran.
The question is formed with why; a direct answer naturally begins with because.
Using it in speech and writing
The choice between because, since, and as mostly concerns register and how new the reason is to the listener, rather than strict grammar: because is more natural in conversation and direct explanations, while since/as are common in correspondence and background circumstances.
Natural speech keeps the subordinate clause of reason complete and does not double it with an unnecessary so or another conjunction: choose one construction for one logical connection.
Since you already have the file, could you forward it to the client?
Since you already have the file, could you forward it to the client?
The polite request relies on an already known fact—a typical use of since in conversation.
The train was delayed because of a signal fault, so we booked a later connection.
The train was delayed because of a signal fault, so we booked a later connection.
Because of + a noun phrase (a signal fault) and so in the next clause are two separate, independent connections, not a doubling of one connection.
Common mistakes
Incorrect:Because of the flight was delayed, we missed the connection.
Correct:Because the flight was delayed, we missed the connection.
Because of requires a noun phrase (because of the delay), not a complete subordinate clause with its own subject and predicate (the flight was delayed)—because is required for a clause.
Incorrect:Because the roads were icy, so the school closed early.
Correct:Because the roads were icy, the school closed early.
Because already links the reason subordinately to the main clause; adding so creates an unnecessary second connector for the same logical connection—keep only one of the two.
Incorrect:As she not answered my messages, I called her instead.
Correct:As she hadn't answered my messages, I called her instead.
For an answer that had not happened before the past call, hadn’t answered is natural. The bare form not answered after she does not create a finite verb form.
Incorrect:Since arrived late, the manager had already left.
Correct:Since I arrived late, the manager had already left.
Since introduces a complete subordinate clause with its own subject—the subject cannot be omitted, even if it seems clear from the context.
Incorrect:Since the office was closed I worked from home.
Correct:Since the office was closed, I worked from home.
When the subordinate clause of reason comes first, a comma is needed before the main clause to mark its boundary.
Incorrect:The reason the shipment was late is that the courier's van break down.
Correct:The reason the shipment was late is that the courier's van broke down.
Broke down is required for a completed past event. The reason is because is also a common grammatical construction; some editorial styles prefer the reason is that, but this is not a universal grammatical prohibition.
Incorrect:As why did the client cancel the order, we called to ask.
Correct:We called to ask why the client cancelled the order.
Because/since/as do not introduce questions about reasons—questions are formed with why, while because/since/as are used only in an answer or a statement.
Incorrect:Since she has moved to Lviv last year, she doesn't visit her parents often.
Correct:Since she moved to Lviv last year, she doesn't visit her parents often.
A definite past time (last year) requires the Past Simple (moved), not the Present Perfect (has moved)—the Present Perfect is typical of temporal since (duration continuing to the present), not of a specific date in a causal clause.