I am just starting a new project which is mostly in purl. The pattern calls for yf (yarn forward). I understand the yf before I do a knit stitch but then it calls for yf between purl stitches. How do I do a yf? The yarn already is in the front. I had to cast on 45 stitches and then after I do the first two rows of all purl I end up with [color=red]42[/color] stitches. I must be doing something wrong with the yf. The pattern is:
p1 p2tog p5 yf (k1 yf p5 p3tog p5 yf) twice k1 yf p5 p2tog p1. PLEASE HELP ME IF YOU CAN!
To YF after a purl, bring the yarn under then back over the needle. 
Silver is right, bring the yarn forward then back as if you have wrapped it around the needle and back to the front. I had a bit of a problem myself the first time I ran across this in a pattern.
Thank you for the yf reply. I will try it and see how it goes. 