Thank you for your help. One more question about the position goes.
1. The cat is in back of the sofa.
2. The cat is behind the sofa.
3. The cat is at the back of the sofa.
3-1. The cat is at the back part of the sofa, on the sofa.
3-2. The cat is not at the back part of the sofa, it is behind the sofa, a little away from the sofa.
[Are the three all the same in meaning? Does #3 mean either #3-1 or #3-2?]
4. The chair is in front of the sofa.
5. The chair is before the sofa.
6. The chair is in the front of the sofa. [ different?]
7. The chair is in the front part of the sofa, on the sofa.
[4 and 5 are the same, but 6 is different. Right? Does #6 mean #7?]
1 and 2 are fine; 3 isn't used.
3-1 -- I'd say, "The cat is on the sofa, near the back."
3-2 -- I'd say, "The cat is about 3 feet behind the sofa."
4 is fine, but I wouldn't use 5 or 6.
7 -- ? The chair is "on the sofa"? Are you sure?? On top of the sofa?
4 and 6 are close, but the word "the" is not needed.
6 and 7 are different; they bring up very different images in my mind!