Spelling rules

http://speakspeak.com/resources/english-grammar-rules/english-spelling-rules/double-consonant-ed-ing

Spelling: when to double a consonant before adding -ed or -ing to a verb

We add -ing to a verb to form its present participle, and -ed to regular verbs to form the past simple. When doing this, we sometimes double the last letter of the verb, as in these examples:

• stop ⇒ stopped, stopping
  stoped
  stoping
• refer ⇒ referred, referring
  refered
  refering

Sometimes, however, we don’t double the last letter, as with the verb visit:

• visit ⇒ visited, visiting
  visitted
  visitting

To understand this spelling rule, it’s first necessary to know the meaning of vowel andconsonant:

vowels = a  e  i  o  u
consonants are all other letters (b  c  d  f  g, etc).

Here’s the rule:

When to double a consonant before adding -ed and -ing to a verb
We double the final letter when a one-syllable verb ends in consonant + vowel + consonant.*	stop, rob, sit	stopping, stopped, robbing, robbed, sitting
We double the final letter when a word has more than one syllable, and when the final syllable is stressed in speech.	beGIN, preFER	beginning, preferring, preferred
If the final syllable is not stressed, we donot double the final letter.	LISten, HAPpen	listening, listened, happening, happened

In British English, travel and cancel are exceptions to this rule: 

travel, travelling, travelled; cancel, cancelling, cancelled. 

* We do not double the final letter when a word ends in two consonants (-rt, -rn, etc.): 
start – starting, started; burn - burn, burned. 

* We do not double the final letter when two vowels come directly before it:
remain – remaining, remained. 

* We do not double w or y at the end of words:
play – playing, played; snow - snowing, snowed.