$\text{1. }$Tom said he had done his homework very carefully the night before.

Quá khứ đơn => Quá khứ hoàn thành

$\text{2. }$Mary said there had been some interesting news about covid-19 recently.

Hiện tại hoàn thành => Quá khứ hoàn thành

$\text{3. }$Mai said her sister had to take part in the army.

Must => Had to

$\text{4. }$Tom said he didn't want to have much homework on his day off

Hiện tại đơn => Quá khứ đơn

$\text{5. }$Peter said his father was working in the garden then.

Hiện tại tiếp diễn => Quá khứ tiếp diễn

now => then

$\text{- Cấu trúc: }$S+ said/ told/....+ (O)+ (that)+ S+ V(lùi thì)

$\text{# Thesun}$