Theorie:

Study the information about the formation of the past perfect.
  
past_perfect_tense.png
  
  
 
Affirmative
subject + had + past participle
You had decided what to do by 5 o'clock yesterday.
decide.jpg
Negative
subject + had + not + past participle
You had not decided what to do by 4 o'clock yesterday.
image1.png
Interrogative
had + subject + past participle
Had you decided what to do by 6 o'clock?
decision.jpg
 
Past participle
  
Irregular verbs
                               
You have to know all the forms of the irregular verbs very well. For the past perfect you need the form of the verb which can be found in the 3rd column of the table of irregular verbs
go – went – gone
                                                                            
Regular verbsadd -ed
work – worked
  
Sometimes there are exceptions in spelling when adding -ed:
  
Consonant after a short, stressed vowel at the end of the worddouble the consonant:
 
stop – stopped
swap – swapped
                                                 
In British Englishdouble one -l at the end of the word:travel – travelled
One -e at the end of the word
add only -d:
love – loved
save – saved
Verbs ending in -y
 
verbs ending in 'y' preceded by a vowel (a, e, i, o, u); add -ed:play – played
 
 
change 'y' to 'i' after a consonant; then add -ed:worry – worried
  
Quellen:
www.haganblount.com
www.grammar-monster.com