A pair of pants has a torn knee that I need to patch up, I put one of those “iron on” patches, but it came off in the wash. Is it a good idea to sew it on? or should I just try to sew the hole closed?

I would take a piece of fabric, or a piece of your patch, a little larger than the hole, and sew it to the inside. Then I would use a zigzag stitch to close the hole, going back and forth with the patch on the back acting as a base.
You'll get a strong mend, but they will probably never do for "nice" again.

Thanks cyndi!! I'm off to sew, sew sew.