turn this into reporetd speech

The first one.

He said, "I shall go to the market" [This is to be turned into indirect
speech]

1 He said he would go to the market

2 He said he should go to the market.

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Number 1 is one choice. But ENLs are not limited and do not always follow the backshifting found in reported speech. Other correct choices, would be;

a. He said that he's going to the market.

b. He said that he'll go to the market.

c. He said that he's going to go to the market.

Sentence number 2 is not really feasible because the shift in meaning.