Study the information about spelling in the past perfect continuous.
  
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Past perfect continuous (spelling tips for adding the ending -ing to a verb):
 
Add -ing to most verbs.
play – playing
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For verbs that end in -e, remove the -e and add -ing.
ride – riding
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For verbs that end in -ie, change the -ie to -y and add -ing.
die – dying 
                                                
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For verbs with last syllables being stressed 'consonant-vowel-consonant' combinations, double the last letter before adding -ing.
beg – begging  
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Signal words: 'all (day, year ...)', 'for (5 minutes)', 'since', '(the) whole (day, week,...)', etc.
 
Examples:
  
They were tired. They had been playing for 2 hours.
  
My friends were happy. They had been riding horses for 3 hours.