1. He couldn't get into the house because he (forget) his keys.
2. When he (write) all his postcards, he had to buy more stamps.
3. He remembered the story very well because he (read) the book several times.
4. By the time he (finish) his studies he wanted a holiday.
5. When I got home I discovered I (lose) my keys.
6. Ryan Patterson came up with his idea after he (visit) a fast food restaurant.
7. He was very hungry because he (not eat) since breakfast.

1)forgot

2)wrote

3)has read

4)had finished

5)lost

6)visited

7)hasn't eaten

вроде так, возможны ошибки

1. He couldn't get into the house because he had forgotten his keys.
In this sentence, we are talking about a past event (not being able to get into the house), which was the result of a previous action (forgetting his keys). To express this in English, we use the past perfect tense "had forgotten." The word "because" shows the cause-and-effect relationship between forgetting the keys and not being able to get into the house.

2. When he had written all his postcards, he had to buy more stamps.
Here, we have another past perfect tense construction. "Had written" shows that the action of writing the postcards was completed before another event took place (having to buy more stamps). The conjunction "when" indicates the time relationship between the two actions.

3. He remembered the story very well because he had read the book several times.
Similar to the previous sentences, we have a past perfect tense construction ("had read") to show that the action of reading the book was completed before the action of remembering. The word "because" indicates the reason why he remembered the story well.

4. By the time he finished his studies, he wanted a holiday.
In this sentence, we use the simple past tense ("finished") to describe the completion of his studies. The phrase "by the time" indicates that the action of wanting a holiday happened at the same time or after the completion of his studies.

5. When I got home, I discovered I had lost my keys.
Again, we have a past perfect tense construction ("had lost") to express the completion of losing the keys before the action of discovering. The conjunction "when" shows the time relationship between the two actions.

6. Ryan Patterson came up with his idea after he had visited a fast food restaurant.
Here, the past perfect tense ("had visited") is used to indicate that the action of visiting the fast food restaurant occurred before the action of coming up with the idea. The word "after" specifies the time order of the events.

7. He was very hungry because he had not eaten since breakfast.
The past perfect tense ("had not eaten") is used here to express the completion of not eating before the feeling of hunger. The word "because" shows the cause-and-effect relationship between not eating and feeling hungry.