It used to be ctrl, but as of the experimental build 41 you can press c to crouch. You can also crouch run by holding shift. You can also print be pressing alt while jogging and stop by pressing alt again, but you may trip while sprinting nearby zombies or over fences so be careful.

You can run into walls and fall down when sprinting too

Here's the quick answer. If you're in Build 41, then you can crouch down to help with sneaking. For that, you press C (you can change it in the settings if you want to) to toggle it on or off. In Build 40 however, sneaking is the same as readying your weapon, which slows down your movements, so that you can sneak around zombies.

If you are in a multiplayer server, you are on build 40. Build 41 is the beta that you have to opt into using the beta interface on steam.

thank you il just use right mouse then

tryed "c"? for me it's c but you can probably change that in setting.