Better sentence-writing in 30 minutes a day - Dianna Campbell 1995

Two notes on spelling
-Ed endings
Revising at the word level

There are two spelling rules that you need to remember.

1. With a verb that ends in y, you usually change the y to i before you add -ed. For example: fry → fried; satisfy → satisfied

2. With a verb that ends in a consonant preceded by a short vowel, you usually double the consonant before you add -ed. For example: trip → tripped; wrap → wrapped