In this level, we add an -ing ending to verbs. In some cases, the final consonant doubles before the -ing ending is added; in other cases, it does not.
A general rule of thumb is if the original verb ends with a short vowel and then a single consonant other than x, the final consonant gets doubled. So run becomes running instead of runing.
Verbs that already end with two consonants (such as kick) or which have something other than a short vowel (such as mail) typically just add the -ing ending as usual with no consonant doubling.