Invalid How do I jump? (Jumping bug when running forward/backward and strafing)

You can't jump if you are simultaneously strafing (or holding the Strafe On key) and going forward or backward.

It's probably that the game can only recognize a certain number of keys at same time. Or maybe it's your computer that can't recognize over a certain number of keys at one time. I've noticed that when I'm playing and pressing a lot of keys down at once, I can't jump either, or one of the other keys doesn't seem to function.(That's my guess, if not then I'm not sure what's wrong).

I thought that might be it at first, but jumping works fine strafing left or right (holding Strafe On so that the same number of keys were pressed).

Swap the keys so you're pressing what you're using for the other combination. Keyboards have a physical hardware limitation on what keys can be pressed at once so that certain combinations don't register. This doesn't mean there's a limit to the number of keys at once, it means specific keys can't be pressed in combation. I can't press Caps Lock, W and G at the same time, but I can press W, D, F, and space at the same time with no issues.

Same on my father's laptop, except only 3 keys at any given time can be pressed, so that makes gameplay a lot harder (except when you are using a mouse).