Good Mornin (or afternoon),
This has come up a lot lately, especially with patterns written in UK terms.
Yf (yarn forward)= if the next st is a knit st is the same as a YO (yarn over)
Yon( yarn over needle)= is usually listed if the next st is a purl and is also a YO.
Yrn (yarn round needle)= YO
Hope this helps ya'll