Many spelling errors are made because the writer does not know whether one or two consonants are needed. As far as verb forms are concerned, the basic rule is that if a suffix such as -ed, -ing, -able and -er is added to the stem, then the final consonant is doubled if it follows a stressed vowel:

If the final vowel is unstressed, then there is no doubling of the consonant: 

There is one exception to the latter part of the rule: in British English, -l is also doubled after an unstressed vowel, though not in American English:

In addition to gaining an active command of the rule, it is also important that you recognize single vs double consonants as a major error type in general. Hence we recommend that you commit to memory as many problem words as possible. Common mistakes include: