1.Dai told Nhi He couldn't go to the library with her the following day.

can→couldn't

tomorrow→the following day

2.Mr Dam said That halloween cake were delicious

this→that 

is→were (Trong câu trần thuật thì không dùng was mà chỉ dùng were thôi nhe)
3.Miss Thao said she had to go then, her people were waiting for her.

now→then

must→had to

4.She asker him What his name was.

5.He asked me how many languages I could speak.

Công thức:
Dạng WH_: S+asked+O(đôi khi đôi khi thì không)+What/When/......+S+V(lùi thì)

Dạng Yes/No question: S+asked+O(khi có khi không)+If/Weather+V(lùi thì)

`1.` Dai told Nhi that he couldn't go to the library with her the next day

S + told + O + (that) + S + V_lùi thì

can't → couldn't

tomorrow → the next day

`2.` Mr Dam said that that halloween cake was delicious

S + said + (that) + S + V_lùi thì

Hiện tại đơn → Quá khứ đơn

`3.` Miss Thao said that she had to go then, her people were waiting for her

must → had to

now → then

Hiện tại tiếp diễn → Quá khứ tiếp diễn

`4.` She asked him what his name was

S + asked + (O) + question word + S + V_lùi thì

Hiện tại đơn → Quá khứ đơn

`5.` He asked me how many languages I could speak

S + asked + (O) + question word + S + V_lùi thì

can → could