How to use crontab editor? How to add new line, how to delete a line or a character? How to save and quit?

Answer 1

When first you run "crontab -e" it asks you for an editor to work with. That may include nano, vim, mcedit, etc...
seemingly you have done it already. To find out what editor exactly you're using try "cat ~/.selected_editor"
Most likely it's vim or vim.gtk3. from here you have two choices, either learn to work with that editor or
run "select-editor" command and choose "nano" which is a simple user-friendly editor.
next time "crontab -e" will invoke what you have selected.
tip: in vi-family editors type ":q" and hit enter to quit.
for more info "man crontab"