Как объяснить правила перевода прямой речи в косвенную. Для начала: глагол главного предложения стоит в наст времени?

Прям. р.: The exam will be difficult. (Present Simple) - > Косв. р.: They said that the exam would be difficult. (Past Simple)

Прям. р.: My English is getting better. (Present Continuous) - > Косв. р.: I knew my English was getting better. (Past Continuous)

Прям. р.: This has been a wonderful holiday. (Present Perfect) - > Kate told me that it had been a wonderful holiday. (Past Perfect)

Прям. речь: Nat grew up in London. (Past Simple) - > I found out that Nat had grown up in London. (Past Perfect)