Carving
How do you carve in the wall in Hammer?
Thanks!
Highlight which block you want other blocks to carve round it and pick tools > carve. Carving should only be used on simple geometry otherwise it will split brushes badly and unnecessarily.
I tried that and nothing happened.
Maybe you should watch a video...
Syp3CW9sFbk
I think the clipping tool is often the better choice.
Clipping simple things with the clipping tools takes a tiny bit more time than the carve tool and complicated things will screw up brushes, as chickenmobile already noted. It can even crash maps entirely. In all the maps I created so far I never actually needed the carve tool.
After dragging the clipping line where you want it, press enter to apply the clip. I also suggest you click the Carve Tool three times before using so that part of the wall doesn't disappear.
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You messed up CARVE and CLIP 
Clipping uses lines and is a good tool to use.
Carve doesn't need lines, you just let a worldbrush go through another one (parent), then you select the worldbrush, press [Ctrl]+[Shift]+[C] And you carve out the shape from your parent. as mentioned before, only use this for simple shapes (squares) because it might screw up your entire brushwork. I prefer to stay away from this tool as much as possible myself. As I said before: Clip can do the same thing too and it's just a lot cleaner, since you have total control over where the cuts will be made 
Ah, crap.
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I'm trying to carve out a hole for a door and an excursion funnel, which are round. How would you suggest doing that?
That is the usual example given of a bad time to carve. It'll screw up your brushes if you do.
I'd recommend clipping around the object, but not deleting anything until you've clipped all the way around the object. Then, delete the middle part and you'll have the hole. Also, to keep your clipping from extending across the entire wall, I'd recommend clipping but not deleting a smaller brush around the object.
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