<em>Because</em> always gives a reason: 'We stopped because the engine failed.' <em>Since</em> can give a reason too, but it also means 'from the time that': 'Since the engine failed, we've been on foot' could be read either way. Use <em>because</em> whenever a time reading is possible.
The core difference is ambiguity, not correctness
Both words can introduce a cause, and causal since has been standard English for centuries. The difference is that since carries a second, temporal meaning that because does not, and readers cannot always tell which one you meant.
Consider: 'Since she took over the department, morale has improved.' Did morale improve as a result of her taking over, or has it been improving in the period following it? The sentence genuinely does not say.
The test
Swap in 'from the time that'. If that produces a sensible sentence, your since is ambiguous — replace it with because if you meant a reason.
Or swap in 'because'. If the meaning survives unchanged and no time reading is available, since is safe. 'Since you're already up, could you get the door?' — no time reading is plausible, so this is fine.
A real difference in emphasis
When both work, they are not identical in effect. Because presents the reason as the new, important information — it is the answer to 'why?'. Since presents the reason as something already known and agreed:
- 'We're rescheduling because the client cancelled.' — The cancellation is the news.
- 'Since the client cancelled, we're rescheduling.' — The cancellation is assumed; the rescheduling is the point.
This is why causal since almost always comes at the front of the sentence, before the main clause. Put it at the end and readers will hear time.
Being honest about the rule
Some style guides — Chicago among them — say plainly that since may be used for cause and that the ambiguity is usually resolved by context. Others, especially in technical and legal editing, ban causal since outright because those fields cannot tolerate any ambiguity. Neither camp is describing a rule of grammar; both are managing risk.
Sensible practice: use because as your default in any writing where precision matters, and use since when you want the softer, 'as we both know' tone and the sentence cannot be misread.
The other 'since'
Do not lose sight of its main job, which is time: 'I've worked here since 2019.' That use takes a perfect tense and pairs with for — 'for six years'. It is far more common than the causal use, which is precisely why the causal use invites misreading.
Frequently Asked Questions
Can I start a sentence with 'because'?
Yes. 'Because the roads were closed, we stayed put' is standard. The only requirement is that a main clause follows, so you do not leave a fragment.
Is 'since' wrong when it means 'because'?
No, it is standard English and has been for centuries. The objection is practical: 'since' can also mean 'from the time that', so it risks ambiguity in writing where precision matters.
Which is more formal?
Neither is markedly more formal. 'Since' sounds slightly softer and more assumptive; 'because' is more direct. Academic and technical writing tends to prefer 'because' for clarity.