If I try to select words in a text editor with Ctrl+Shift+Left/Right, or try to copy text from a terminal with Ctrl+Shift+c, it doesn't work. How can I fix this? I use KDE.
There is no reason why it should work in console, since console is not a text editor. In console, you have different priorities.
But if you really want to select text in the console using keyboard, it is possible to do so in
tmux, which is described in this related questions.