Because we stopped feeding dog food to our pet turtle, he's much livelier.
Since we stopped feeding dog food to our pet turtle, he's much livelier.
Which one's right?
Answer? Both. But here's the thing: they mean two different things:
- Because indicates a causal relationship. It's like saying "We stopped feeding dog food to our pet turtle, and because of that, he's much livelier."
- Since indicates a temporal (time-related) relationship. It's like saying "Our pet turtle has been a lot livelier starting from the time we stopped feeding him dog food."
To be honest, lots of people use because and since interchangeably, and we're guessing no one's going to slap a Scarlet G (for grammar) on you any time soon if you do it, too. But if you're taking a grammar quiz, you best know the difference.
Why is this here? Well, one can be a preposition and one is never ever a preposition. One thing we definitely know about prepositions is that they must have objects (or at least implied objects). To figure out which is a preposition, let's put a noun phrase after both of them and see which makes sense:
She hasn't laughed since the accident.
*She hasn't laughed because the accident.
Oops. She hasn't laughed because the accident what? Because it traumatized her? Looks like "because" isn't a preposition. Make sure you know which one you're using.