When you take all the time and effort required to knit a sweater, it can be heartbreaking if something happens to damage that sweater. If you saved some leftover yarn from your knitting project, you will be able to use it to repair the sweater; otherwise, try to purchase yarn that matches the sweater's colour and fibre content as closely as possible. No matter where the problem occurred, you should be able to repair it or cover it up to make the sweater wearable again.
Unravel the sweater slightly where it has torn or unravelled itself, so that you can see the loops of the stitches from the hem or sleeve of the sweater.
Thread a knitting needle through the stitch loops on the sweater's hem or sleeve. If the problem occurred around the entire sleeve or hem, you will need to use circular or double-pointed needles (DPNs) to pick up these stitches.
Pick up a skein of coordinating or leftover yarn and knit the stitches you picked up in step 3. Follow the knitting pattern established in the sweater; it will probably be some sort of ribbing, such as knit two/purl two/repeat.
Bind off the stitches when you have made the hem or sleeve as long as it needs to be. Cut your working yarn with scissors and thread the yarn tail onto an embroidery needle, weaving it in on the inside of the sweater to hide it.
Pull out the extra yarn tails around the hole or tear in your knitted sweater, being careful not to unravel the hole too much further.
Thread a knitting needle through the stitch loops at the top of the hole.
Pick up a matching or leftover skein of yarn and knit the stitches you picked up in step 2. Knit back and forth in the established stitch pattern of the sweater. Stop knitting and bind off your stitches when you have knit a piece large enough to fill the hole.
Thread some extra yarn onto an embroidery needle. Use this needle to sew the patch you made into the hole. Sew the bottom edge of the patch to the bottom stitches, threading the needle and yarn through those stitches to avoid unravelling. Then sew the sides as well; keep your stitches on the underside of the sweater for the most part to keep them hidden.
Sew a premade patch over the hole as an alternative if the hole is very large. Make sure to sew tightly around the perimeter of the patch to keep the knitting stitches from unravelling.