"So Far" or "Yet" ?
You haven't given us much to go on! Can you post a couple of sentences where you would like to know which to use, or if they are interchangeable?
.... Since I have not heard back from him so far ..... Yes
.... Since I have not heard back from him yet ..... Yes, though this is better
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.... as yet we have received 25 complaints ..... No. As yet at the beginning of a sentence is normally followed by a negative - "As yet, I haven't spent any money, but I will later.
.... so far, we have received 25 complaints ..... Yes