After many years in the United States I still haven't figured out the difference in usage between "as" and "since." What I do know is that both denote (a usually well-known) "because," so most of the time I can substitute "because" and get away with it. But there are times when that either doesn't work or I would have to start a new sentence, which would not be appropriate in the context. Google searches so far have yielded nothing useful. Would someone please explain the correct usage of, and/or give a rule-of-thumb for when to use, "as" vs. "since." Thank you.
Both the words are used as conjuction, preposition and advrb. The word ‘since’ is used to express time as adverb and preposition, as below.
I had seen him previously, but hadn't seen him since. (adv)
I have known her since last year. (pre)
The word ‘as’ is used to express extent/degree as adverb and preposition; as below:
You’re heavier than I am, and not as tall. (adv)
You are not as tall as I am. (pre)
However, as conjunction they are usually used introducing reasons with verbs of the senses (feel, hear, read...), expressing emotion (admire, hate, love, like...), In this case they are interchangeable.
As it’s too late, I quit or Since it’s too late, I quit
As/Since we read, we learn. Since/As we were in the computer lab, our English has improved
In certain situatations ‘as’ can only be used; as:
As I came in, she flew.(when)
He sleeps as the rain falls (while)