The remaining sections focus on more desireable and convenient commands. If a typing mistake needs to be corrected and you are on the entering text on the line in question. The different implementations of vi behave differently. Some just move the cursor back and the text can still be viewed and accepted. Others will remove the text as you backspace.
Some clones even allow the arrow keys to be used to move the cursor when in input mode.
This is not normal vi behaviour. Use your editor to become accustomed to its' behaviour. Delete one character from under the cursor dw Delete from the current position to the end of the word dd Delete the current line. The modifiers can be used to add greater power to the commands. The following examples are a subset of the posibilities.