and add (ed) or (ing).
To state this rule simply; “short vowel, one consonant, double” (It needs a friend)
Example: The letter “u” is a short vowel in the word run. It is followed by one consonant (n), therefore the last letter (n) is doubled - running.
If the short vowel is followed by two consonants (mp), as in the word jump, the last consonant is not doubled - jumping.