1. This is the hospital. He was born in the hospital.
2. This is the hospital where/that he was born.
3. This is the hospital he was born.
4. This is the hospital in which he was born.
(Are they all grammatical? Can we use both 'at' and 'in' before 'the hospital'?)
5. Tell me the reason. He was absent from school for the reason.
6. Tell me the reason why/that he was absent from school.
7. Tell me the reason he was absent from school.
8. Tell me the reason for which he was absent from school.
(Are they all grammatical? Do I need any correction? Thank you for your help.)
#1. Both sentences are correct, but don't quite match. "He was born in THIS hospital" would be better. As stated, it could be any hospital.
#2. Where I better than "that".
#3. Not correct. #4 IS correct.
#5. The first sentence is correct. The second doesn't quite make sense.
#s 6 - 8 are correct. In #6, either "why" or "that" are okay.