When are “because”, “since”,“for” and “as” interchangeable?
A few years ago, I was told that "since" should only be used to designate a time period. For example:
Since 2 o'clock, I've been waiting for you.
However, since creeps into the place of "because" quite often.
Since I don't have the time, I won't be joining you.
This second example sounds slightly slangy to me now that I've been avoiding using "since" in this way for the past few years, but is "since" really a synonym of "because"?