Every knitter will eventually drop a stitch. Sorry, but it's apparently a law of Nature. This doesn't have to be a disaster, however. As in the rest of life, planning ahead is the answer. Practicing "the fix" starts with making mistakes on purpose. On big needles with bulky, clingy yarn make a stockinette swatch. Locate a crochet hook - any size will do, as long as it's not big enough to stretch your stitches.
Dropping Stitches on Purpose
After a few inches, deliberately drop a stitch in the middle of a right-side row. Don't just take the stitch off the needle; actually undo the running thread from the row before. The running thread is the yarn parallel to the needle between the stitches. It will come out from the back, leaving a stitch which was completed 2 rows ago. Put this stitch back on the left-hand needle and take a good look at it.
Notice that the running thread from the row before goes un-worked behind this stitch. If a stitch is accidentally slipped, this is what it will look like on the next row, and that's the best place to fix it. When stitches are dropped off the needle, they feel obligated to unravel even further leaving a bigger gap. Experiment with that too, but first get a handle on this smallest of problems.
Stockinette: the Easiest Stitch to Restore to Grace
Here's the thing about stockinette. When the right side of the work (the side where every stitch is knitted) is facing the knitter, the crochet hook goes through the stitch from front to back (away from the knitter), and the running thread is behind the stitch. Get the stitch on the hook, hook the running thread. Then, pull the hook out (toward the knitter). As the hook comes back, it draws the running thread through the stitch. Another way to look at it is that the newly picked up stitch passes over the hook, which is still holding the running thread. Try helping this along with the fingers to get comfortable with what's happening.
The result is that the running thread will now be the new stitch. Transfer it from the hook back to the knitting needle. To do this, point the hook away from the knitter like before. Then, use the left-hand needle to pick the stitch from the hook; go under the stitch so that the needle is pointing in the same direction as the hook.
The reason for starting with stockinette is that this procedure is done the same whether the stitch was dropped from a right-side or wrong-side row. Always turn the work so that the right side is facing you. Enter the stitch from front to back and pick up the running thread which will always be behind the stitch, unless it's been twisted. If the running thread isn't back there, it should be, so untwist it to get it where it belongs. It is worth practicing picking up other types of stitches, but get it right on stockinette first.
Digging Deeper into the Knitted Past
After mastering dropped stitches from a row back, take out another row. Learn to separate the two running threads. Hook the one that's further down first. Picking up stitches won't always be good enough. If a stitch is dropped in the Stone Age (say, ten rows back), there may not be enough running thread in later rows to fix it without an unpleasant pucker. That's where using placemarkers and careful stitch counts can make all the difference. The remedy for these problems is "tinking" (knitting spelled backwards or "unknitting") and/or "frogging" (just rip it out), but let's leave that to another day.
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