[Grammar] He said that he MUST DO/MUST DID it. (Do modal verbs change according to the tense?

[Sahil Dhankhar]

This is not my homework.

He says, "I must do it".
How will we write it in indirect sentence? -

He said that he must do it.
or
He said that he must did it.

Similarly-
He said to me, "You should do it".

He said to me that I should do it.
or
He said to me that I should did it.

Similarly-
He said, "I shall go there".
He said that he should go there.
or
He said that he should went there.

 

[Matthew Wai]

"Must do", "should do", and "should go" are grammatical.
"Must did", "should did", and "should went" are ungrammatical.

 

[Rover_KE]

Modal verbs are followed by the bare infinitive: do.