Grammar usage

Since as an preposition

Since is most commonly used as an preposition, with Perfect tenses, to describe a point in time when some action began.

I have lived in Tokyo since 2009.

I've been to Kyoto three times since I moved to Tokyo.

She's been working on her essay since 9 am this morning.

Yesterday Joe phoned at noon to invite me out for lunch. I'd been working on my essay since 6 am, so I decided to take a break and join him.

Since as a preposition must be used with a Perfect tense.  Because cannot be used for this meaning and it is not a preposition.

Since as an adverb

Since can be used an adverb, and expresses one of the following three meanings: 

1. (especially with ever) from then until now:  They met in high school and have been a couple ever since.

2. between a particular point in time in the past and now:   At first, she refused to marry him, but she has since accepted his proposal.

3.  ago; before now:  I have long since given up on every marrying.

Because cannot be used for this meaning.

Because and since as conjunctions

Because is always a conjunction that explains the reason for something. According to Michael Swan (Practical English Usage, 3rd ed., p. 67), and other sources, it is the conjunction to use when you want to "put more emphasis on the reason, and [it] most often introduces new information which is not known to the listener/reader."   

On the other hand, Swan and others stipulate that since is the conjunction to be used to introduce a reason that is already known/understood by the listener/reader, and the speaker/writer wants to focus on the result or consequence.

At first, I thought this stipulation/differentiation was silly, but in fact the more I have thought about it, and how I use these conjunctions, the more I agree with it.  Please consider these example sentences: 

I went to high school in Toronto because my father got transferred there.  (since would be incorrect, or at least really, really unnatural here; the listener/reader doesn't know why, or the statement would be completely unnecessary.  The sentence answers the question "Why did you go to high school in Toronto?"  The reason is what is important.)

Since you're here early, let's get started.  (The listener already knows they are here early; because in this sentence would sound unnatural, if not incorrect. The result or consequence is what is important.)

It is important to realize that some grammarians consider the use of since as a conjunction to always be informal; some, to the point of it being unacceptable.  I completely disagree; the above distinction is an incredibly useful one, and reflects how I speak and write. However, you do need to be aware that the strict grammarians will warn against ambiguous usages, where the reader/listener has trouble figuring out whether you are using since as a conjunction or as a preposition.  See the examples below, along with my militant rebuttals thereto.

So-called ambiguous usage of since

Because since can function grammatically either as a conjunction or a preposition, some grammarians bolster their argument against using it as a conjunction because, they say, certain sentences can sound ambiguous.  Consider this sentence: 

In a second term, Carter might have moved the course of government toward the left, but since Reagan won the election the nation’s political movement has been toward the right instead.

In this sentence, since could be read as a conjunction meaning because or it could be read as a preposition meaning from that point onwards.  Anyone with a grasp of recent American political history would know that the intended meaning is because; given the potential ambiguity, the sentence could be reworded to remove it:   In a second term, Carter might have moved the course of government toward the left, but as a result of Reagan winning the election, the nation’s political movement has been toward the right instead.

However, I think that in the above sentence, the difference between a causal conjunction or a durative preposition is almost not worth talking about.  I have yet to see an example sentence with since as a causal conjunction that would be truly, completely, madly ambiguous.

I've been lonely since you went away.  Does it mean that I am lonely because you went away or from the time that you went away?  Almost anyone would interpret it as because, but is there a meaningful difference between the two interpretations that wouldn't be otherwise completely clarified by the context, especially considering the nuances--which is more important, the reason or the consequence?--outlined above?  If there is, such differences don't keep me up at night.  I will eat my hat if shown an example sentence that is blatantly ambiguous.

I hope this helps.