lvirtuozlrus, In your sentences "yet" and "as yet" have the meaning of "up to this time". With this meaning "yet" is used in negative sentences and questions. -We've had no luck yet. -Have we had any luck yet? "As yet" and "as of yet" are used in formal and literary English. That is the difference. We haven't heard from him yet. = We haven't as yet heard from him.

'Yet' is an adverb meaning the following: 1. At this time; for the present: isn't ready yet. 2. Up to a specified time; thus far: The end had not yet come. 3. At a future time; eventually: may yet change his mind. 4. Besides; in addition: returned for yet another helping. 5. Still more; even: a yet sadder tale. 6. Nevertheless: young yet wise. Yet as a conjunction means:. And despite this; nevertheless: She said she would be late, yet she arrived on time. 'As yet' means up to the present time; up to now. Example: As yet I have not won the lottery.