Question

he had not returned yet which tense is used​

Answer 1

Answer:

Hope this will help you

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Explanation:

The librarian asked him why he had not returned his book the previous day is more suitable due to certain conditions which need to be fulfilled under reported speech. Once the sentence has already begun in past tense, the reported part goes into the past participle, hence did not return is the simple past and is wrong.

Answer 2

Answer:

May be its Simple past tense.