ted
Editing text
The editor works as you expect from any text field, with a few things of its own: indentation follows the file, and pasted text takes the file's line ending.
Moving and selecting
The arrows move by character and line, Home and End to the start and end of the line, Page Up and Page Down by a screen, and Ctrl Home and Ctrl End to the start and end of the file. Hold Shift with any of them to select. Ctrl A selects everything.
With the mouse: click to place the cursor, drag to select, Shift click to extend the selection, double click to select a word and triple click to select a line.
Clipboard and undo
Ctrl C, Ctrl X and Ctrl V copy, cut and paste. With nothing selected, copy and cut take the whole line. Pasted text takes the line ending of the file. Ctrl Z undoes, and Ctrl Shift Z or Ctrl Y redoes; typing a word is one step.
Indentation
Tab indents and Shift Tab outdents: the cursor to the next stop, or every selected line by one level. The level follows the file: a file indented with tabs gets tabs, a file indented with two spaces gets two spaces. In a new or flat file, the Tab size and Insert spaces settings decide. The status bar shows what the file uses.
Line endings and encodings
ted keeps the line ending of each file, LF or CRLF, and the byte order mark of a UTF-8 file that has one. A file that is not valid UTF-8 opens read only, its unknown bytes shown as replacement characters, so saving can never change bytes ted could not read. Binary files are not shown.