Put the (verb) in the Past Perfect form
He couldn’t get into the house because he ____(forget) his keys
He couldn’t get into the house because he had forgotten his keys
After he _____(write) all his postcards, he had to buy more stamps.
After he had written all his postcards, he had to buy more stamps.
He remembered the story very well because he ______(read) the book several times.
He remembered the story very well because he had read the book several times.
He took a long vacation after he _______(finish) his studies
He took a long vacation after he had finished his studies
When I got home, I discovered I ______(lose) my phone.
When I got home, I discovered I had lost my phone.