Теория:

Study some signal words for Past Perfect.
 
  
Words for Past Perfect:
  
before — перед тем как, до;
 
after — после;
 
already — уже;
 
just — только что;
 
for — в течение;
 
since — с тех пор;
 
till/until — до тех пор;
 
by the time — к тому времени;
 
never — никогда;
 
earlier — ранее;
 
when — когда;
 
yet — ещё не.
 
 
Examples
 
She knew how to cook a cake because she had made it earlier
  
cake-1850011_640.png
Рис. \(1\). Cake

James easily won this game because he had played it many times before
  
children-593313_640.png
Рис. \(2\). Winning a game

John had sat to work when his boss called for him. 

We had already bought a cake when Jill called us and told that she bought a cake too. 
  
I hadn’t served the table yet when the guests came.
Источники:
Рис. 1. Cake, 2016, https://pixabay.com/images/id-1850011/ Pixabay License CC0 Free for commercial use No attribution required
Рис. 2. Winning a game, 2014, https://pixabay.com/images/id-593313/ Pixabay License CC0 Free for commercial use No attribution required