I don't really have nails because I bite my fingers so it's quite difficult for me to undo some knots.
What is an easy way to untie knots when fingers just can't do the job?
Referring to knots in fabric, string, and other such things... Anything not a polythene bag as it makes all the difference.
First, get a spoon (preferably wooden). Bang on the knot, this will loosen it up. Now you can do one of two things. I prefer to use a corkscrew, but you can also use tweezers or pliers. For the corkscrew, put the point in between the strings of the knot. Twist and wiggle the corkscrew and eventually the knot will come undone. You can follow a similar procedure for tweezers and pliers.
Image from Wikipedia
All knots rely on stress between the turns on the rope to stay together. If you are able to release some of the tension so it is easier to untie the knot.
Tapping on it can help, and doing something similar to the plastic bag solution can help. That is to twist the ends and push them into the knot to loosen it. Also locating a bend which is movable can help releave tension, and jiggle this around a bit.
Finally you can apply pliers or flat screw drivers in some bend to help it loosen from the other parts, but be careful to actually loosen it, and not tightening it.
Scissors work amazingly. No, I'm serious. If you don't care too much about the knot, just cut it with a pair of scissors.
But I would also focus on the bigger picture: biting your nails. I struggled with this as well, and I've found that there are two main situations where I find myself biting my nails: 1. when they're jagged, and 2. when they're too long.
Buy yourself two good nail clippers and leave one at home, and carry the other with you. First of all, clip your nails to be as neat as possible, so that you have no jagged edges to tempt you into biting them. When your nail breaks in the middle of the day, instead of biting them (which only makes it worse), use the nail clippers you carry with you to solve it. And finally, get into the habit of clipping your nails often so that you're clipping them instead of biting them.
There is a tool made just for your problem. It's called a 'marlinspike' and is used by sailors to undo knots and bends when they've been drawn too tight.
You can find an example here.
As others have stated, the best bet is to attempt to add material into the knot/bend. This facilitates loosening the troubled area.
To explain further, from an end of the work, find the first crossing and, from the side opposite the end, insert the marlinspike (or corkscrew, screwdriver, what-have-you) between the end piece and the crossing (typically, under the crossing piece toward the end.)
This should allow you to make a little extra space in the first crossing's loop and then you can push some material from the end into the knot, thereby creating more slack in the knot / bend.
Continue to do this with all the crossings in the knot/bend until satisfied with the result.
Work a strong narrow/thin object under one part of the knot, like a screwdriver, plier, or even bamboo skewer or knitting needle. Then carefully wiggle it to work the knot loose. Repeat on different parts of the knot if necessary.
Try soaking the knot in water and then placing it in a microwave. Make sure there isn't any metal or that it won't give off noxious fumes when heated. Alternatively try lubing it with some kind of oil and putting it in the freezer.
