字尾的問題(double)

Please refer to the spelling rules for adding "ed".
http://www.really-learn-english.com/spelling-rules-add-ed-verb.html

"Double" mostly occurs after the words with the second last letter which is a vowel.

prefer
past tense變preferred
但係flower
past tense變flowered
請問究竟DOUBLE R

= no rules but we have to learn them by heart - most fer with rred

travelled
fulfilled
preferred
referred
deferred

to write "ed" the past tense is not always have the rules!