How to cut off part of an object?

Question

Blender

It is necessary to exactly cut off the bottom of the object:

I did not find how to do this. In Wireframe mode, the mesh does not appear.
File from here .
Tried holding Shift while the Knife tool is selected, didn't help.
You can probably do it in a couple of clicks.
Is the Y button in the screenshot pressed or pressed?
I wanted to expand the view on the right to full screen using Ctrl + arrow up - it does not work. How to deploy?

Answer 1

At the knife, instead of holding the shift, you need to turn on the binding C , and also the through cutting Z
Y is pressed. In the default theme, pressed buttons are dark gray.
In knife mode, maximizing the window does not work.