1. Dad is in back of the door. [O]

2. Dad is behind the door. [O]
3. Dad is at the back of the door. [?]
4. Dad is in front of the door. [O]
5. Dad is before the door. [O]
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I'd like to ask you the question one more time.
Some native speakers seem to use #3 and some don't think #3 is correct.
Does #3 mean #1 or #2? Or is #3 totally different from #1?
Would you clear that one more time. If a student use #3 to mean #1, is it grammatical?

6. I hit the chair in front of me. [O]
7. I hit the chair in back of me. [O]
8. I hit the chair at the back of me. [?]
9. I hit the chair in the back of me. [X]
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Is #8 not the same as #7?

I don't know any native English speakers who would use 3, 5, 8, or 9.

1 and 2 are clear, and there doesn't need to be any substitution. Same with 6 and 7.

Let's analyze the sentences to understand the meanings and grammaticality of each one:

1. Dad is in the back of the door. [O]
2. Dad is behind the door. [O]
3. Dad is at the back of the door. [?]
4. Dad is in front of the door. [O]
5. Dad is before the door. [O]

In sentences 1 and 2, "in back of" and "behind" are both idiomatic expressions that mean the same thing. So, both sentences 1 and 2 are grammatically correct and have the same meaning. Sentence 3, "Dad is at the back of the door," is less commonly used and may be considered less idiomatic. The usage of "at" in this context might suggest a location directly adjacent to the back of the door without necessarily implying physical proximity to the door itself. Due to its less common usage, sentence 3 can be confusing or unclear to some speakers, and the exact meaning may vary depending on the context.

Regarding sentence 8, "I hit the chair at the back of me," it is less common to use "at" in this context. The more idiomatic expression would be "in back of" as in sentence 7, "I hit the chair in back of me." Using "at" in this context, as in sentence 8, can make the sentence sound awkward or unclear. It's not grammatical in the standard sense because "at the back of me" is not a commonly used expression to convey the intended meaning. Sentence 9, "I hit the chair in the back of me," is not idiomatic and does not convey the intended meaning clearly.

To summarize, while sentences 1, 2, and 7 are grammatically correct and convey the intended meaning clearly, sentences 3, 8, and 9 may sound less idiomatic or could be confusing to some speakers. It's important to use more commonly understood expressions to ensure clear communication.