It is very, very important that you use the -w switch when opening a config file. Failure to do so may keep your system from booting or cause other bad things.

Help
----
CTRL+G
F1

Save
----
CTRL+O
F3

Exit
----
CTRL+X
F2

Cut (also delete)
---
CTRL+K

Paste
-----
CTRL+U

Delete
------
CTRL+K #line
CTRL+D #character under cursor
also Backspace and Delete keys will work

Search
------
CTRL+W

Search and replace
-------------------
CTRL+\

Display current cursor position
-------------------------------
CTRL+C

Start nano with displaying line numbers
---------------------------------------
nano -c file

Start nano e.g. from line 20,character 30:
nano +20,30 file
nano +20 file #only line
nano +,30 file #only character

Go to line, character
---------------------
CTRL+_
ALT+G
... "M" (Meta) means usually ALT key

Start nano with automatic backup (using suffix ~) when saving a file:
nano -B file
ALT+B toggle backup mode

Enable undo, redo (claims to be a bit experimental)
---------------------------------------------------
undo: ALT+U
redo: ALT+E

View only
---------
nano -v file
nano --view file

Wrapping
--------
no wrap:
nano -w file
nano --nowrap file
ALT+L toggle

soft wrap:
nano -$ file
nano --softwrap file
ALT+$ toggle

Work with multiple files
------------------------
nano -F file1 file2
or
nano -F file1
... and open rest of files with CTRL+R
... move between opened files:
ALT+, (backward)
ALT+. (forward)

ALT+F toggle mulibuffer mode

go to the beginning of text and start marking
CTRL+^

mark text and to copy it to RAM:
ALT+^

^ comes with ALTGR+Ä on Estonian keyboard

Go to the next file and paste in right place:
CTRL+U
... and save changes

Some toggles
------------
ALT+P #whitespace
ALT+X #help mode

Links
-----
https://www.nano-editor.org/dist/v2.2/nano.html
https://wiki.gentoo.org/wiki/Nano/Basics_Guide
https://stackoverflow.com/questions/19816699/how-to-open-multiple-files-and-switch-between-them-in-nano-text-editor
https://superuser.com/questions/548447/copying-from-one-file-to-another-using-nano-editor
https://askubuntu.com/questions/73444/how-to-show-line-numbering-in-nano-when-opening-a-file