Find the part with the error: The Chief Secretary / explained us / the new policy / in detail.
- Subject and verb agree, and the simple past is a suitable tense, so move to the verb pattern.
- 'explain' takes the thing explained as its object and needs 'to' before the person.
- 'the new policy' and 'in detail' are both correct.
Answer: The second part, 'explained us'; it should read 'explained to us'.