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I wonder if there is any rule for doubling the p at the end of a stem. For example:

stop — stopped

but

help — helped

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marked as duplicate by Mari-Lou A, Edwin Ashworth, Janus Bahs Jacquet, TimLymington, StoneyB Dec 22 '13 at 13:02

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The easiest rule to remember is: when a word ends with a consonant, preceded by a single vowel, then that consonant is doubled in the past simple tense. But in American English if a word ends with a single "l" it is left alone. BrEng travelled AmEng traveled –  Mari-Lou A Dec 22 '13 at 9:48
    
@Mari-LouA thank you for short description, it makes sense –  Maxim Shoustin Dec 22 '13 at 9:51
    
Perhaps you would like to visit ELL English Language Learners our sister site? –  Mari-Lou A Dec 22 '13 at 9:53
    
@Mari-Lou: when you answered, you should have said there were numerous exceptions. For another example, compelled. There is a more complete rule which says that for multi-syllable words in American English, you double the final letter only if the last syllable ends with a consonant, contains a single-letter short vowel, and is accented. But again, it has exceptions. –  Peter Shor Dec 22 '13 at 11:32
    
So for OP's 'What is the rule' read 'Is there anything we could even term a rule of thumb . . .'. –  Edwin Ashworth Dec 22 '13 at 12:09