1. Dad is in front of the room.

Question

2. Dad is in back of the room.
3. Dad is in the back of the room.
4. Dad is at back of the room.
5. Dad is at the back of the room.
[Which ones are the opposite of Sentence 1? Do we have to use 'the' or not?]

Answer 1

You should use "the" -- yes.

The best phrased opposites of #1 are #3 and #5.

Answer 2

Thank you.

6. Dad is in front of the door.
7. Dad is in back of the door.
8. Dad is in the back of the door.
9. Dad is at back of the door.
10. Dad is at the back of thedoor.
[What about these sentences? Which ones are the opposite of Sentence 1? Do we have to use 'the' or not?]

Answer 3

With this phrasing, you would NOT use "the"!!

So 6 and 7 are the acceptable sentences here.

Answer 4

9. Dad is at back of the door.
10. Dad is at the back of the door.

[I appreciate your help. What about #9 and #10?]

Answer 5

Neither of those is quite right.

Dad is in back of the door. (meaning he's behind it)
Dad is in front of the door. (meaning whoever else is in the room can see him there)

Using "the" is not correct for this particular concept.This is different from being in the front of the room (think of a classroom in which all the students are facing him) or in the back of the room (students still facing the front of the room, but he's now behind them).