We normally add -ing to a verb to form its present participle, and -ed to to form its regular simple past. When doing this, we sometimes double the last letter of the verb, as in these examples:
• refer ⇒ referred, referring
• stop ⇒ stopped, stopping
Sometimes, we don’t double the last letter, as with the verb visit:
• visit ⇒ visited, visiting
So when can we double a consonant before -ed or -ing to a verb exactly? Take a look at the table below:
|We double the final letter when a one-syllable verb ends inconsonant + 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.
||beginning, preferring, preferred
|If the final syllable is notstressed, we do not double the final letter.
||listening, listened, happening, happened
• We do not double the final letter when a word ends in two consonants:
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.