The present continuous tense is formed by adding "-ing" to the base form of the verb. Sometimes the base form of the verb needs a spelling change before adding "-ing," but doubling the last letter is not one of them.
Here are some examples:
* walk + ing = walking
* play + ing = playing
* swim + ing = swimming
The spelling changes that occur are:
* Adding -e to verbs ending in -y: cry + ing = crying
* Dropping the final -e before adding -ing: write + ing = writing
* Doubling the final consonant before adding -ing: run + ing = running
Remember, doubling the final consonant only happens when the verb is a one-syllable verb ending in a single consonant preceded by a single vowel.