過去形と過去完了
皆さんはじめまして。
過去形と過去分詞の問題集を解いているのですが、下記の問題がいまいちわかりませんので、ご教示頂けますと幸いです。
問題指示文：
Underline the correct answers. In some cases only one is correct, and in others both are correct.
問題１
As Geoff was introduced to Mrs Snape, he realised that he [had met/ met] her before.
回答：had met
問題２
During the previous week, I [had been/ went] to the gym every evening.
回答：Both
問題３
He denied that he [had taken/ took] the money from the office.
回答：Both
問題４
The boy told me that he [had lost/ lost] his train ticket and didn't know how he would get home.
回答：had lost
問題５
The teacher guessed that some of the children [had cheated/ cheated] in the exam.
回答：Both
問題１のみ理解できるのですが、その他の問題がいまいちわかりません。もちろんすべてにご回答頂かなくても問題ございませんので、なにか法則や共通点等をご教示頂けますでしょうか？
宜しくお願い致します。
