Yarn Forward (yf)
How do you bring a stitch forward and then what do you do with it?
Yarn forward (yf) or Yarn over simply means bringing working yarn toward the front (not a stitch). You do not need to do any thing with a stitch at this point until you see the next instruction.
Making YF by bringing working yarn that is in the back of work into the front of work and continue on with the pattern.
k1, yf, k1 means knit a stitch, bring the yarn forward (or to the front) just as for purl, knit the next stitch while the yarn is still in the front. This way the yf will turn into an eyelet making an extra stitch.
I hope this help and thanks for asking.