Here's a little nit that hardly ever gets picked any more. In popular parlance, since and because have become more or less interchangeable. However, they don't actually mean the same thing.
Since implies a temporal relationship between two factors. The item in the dependent since clause (or phrase -- since can be used in either) sets the moment at which the topic of the independent clause became true. It also describes an ongoing situation or event that has been roughly continuous from the moment of the since until the "present" of the main clause.
Since the day we met, I've been able to think of no other man but you.
In other words, something changed, and that change can be traced to the since moment, and that change has been constant from the moment of the since. You wouldn't write,
Because the day we met, I've been able to think of no other man but you.
Because implies a causative link between the independent and dependent clauses. The item in the dependent because clause led to the existence of the item in the independent (main) clause. Get it? Because = cause.
Because the temperature dropped below freezing, all my tomato plants died.
The temperature change led directly to the death of the tomato plants. The death of the tomato plants is a one-time event, not ongoing or evolving.
In the common tongue, people frequently say,
Since the temperature dropped below freezing, all my tomato plants died.
And loads of perfectly respectable, well-educated folks don't shudder at the horror of the botched adverb. I know, I know. Hard to believe. But there you have it. And those of us who know better -- which now includes you fine readers -- have to accept that the language changes over time.
So when is the difference between since and because relevant or irrelevant in your writing? My rule of thumb is to correct it outside the quotation marks. Inside quotation marks, the difference can be ignored unless the character would not ignore it. Lawyers, for example, with their obsessive addiction to causation and the types of links between things, understand the difference between since and because. They're trained in it. Their speech is more likely to reflect that training. So if you have a legal thriller under way, you might search your manuscript on the keyword since and fix it throughout. What other types of characters would preserve this difference?
I'm tempted to go on a mini-rant here about why precision in word choices is so crucial to good writing, but it's Monday morning, and who needs that on a Monday? None of us. So instead, I'll treat myself to a fresh cuppa and skip the rant.
January was bedlam for me this year. So glad it's February. It's lovely to have a moment to commune with the blog. :)