1. The boy is in the hospital.

2. The boy is in hospital.
3. The boy is playing in the hospital.
4. The boy is hospitalized for illness.
5. The boy came to the hospital to visit his grandfather. He is playing with his brother near his grandpa.
6. He is at the hospital.
7. He is at hospital.
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Does #1 mean #4 or #5?
Does #2 mean only #4?
Does #6 mean #5? Is it the same as #1?
Is #7 possible?

1 is correct in US English.

2 is correct in British English.
3 is correct.
4 would be correct if you put "an" in front of "illness."
5 is correct.
6 is correct.
I've never heard/read anyone using 7's phrasing.

I'd say that 1 and 2 both mean the same as 4.

Yes, 6 and 5 mean the same thing.

Thank you for your help.

When the boy is not sick and just came to visit his grandpa, can we say,'The boy is in the hospital?'

To make it clear, you might say, "The boy is visiting ... in the hospital."

You're very welcome!

To understand the differences between the sentences, let's analyze each one individually:

1. The boy is in the hospital.
This sentence implies that the boy is a patient and is currently receiving medical treatment or care at the hospital.

2. The boy is in hospital.
In some varieties of English, including British English, it is common to omit the definite article "the" before certain places, such as hospital. Therefore, sentence 2 is equivalent to sentence 1 and also means that the boy is a patient in the hospital.

3. The boy is playing in the hospital.
This sentence suggests that the boy is not a patient but is instead engaging in activities or games while in the hospital. He might be a visitor or someone who works at the hospital.

4. The boy is hospitalized for illness.
In this sentence, it is implied that the boy has been admitted to the hospital specifically because of an illness or medical condition. It does not provide information about whether he is playing, visiting, or receiving treatment.

5. The boy came to the hospital to visit his grandfather. He is playing with his brother near his grandpa.
Sentence 5 indicates that the boy is not a patient but has come to the hospital to visit his grandfather. While he is there, he is playing with his brother near his grandfather.

6. He is at the hospital.
Similar to sentence 1, this sentence suggests that the person (whose gender is not specified) is present in the hospital, but it does not provide information about their specific purpose or role.

7. He is at hospital.
In standard English, it is more common to use the definite article "the" before the word "hospital." Although sentence 7 may be acceptable in some non-standard or colloquial varieties of English, it is generally considered less grammatically correct than using "the" before "hospital."

Based on the explanations given, sentence 1 and sentence 4 convey a similar meaning as they both imply that the boy is a patient in the hospital. Meanwhile, sentence 2 can also be interpreted to mean the same as sentence 1. Sentence 3 suggests that the boy is engaging in activities while in the hospital, and sentence 5 explains that he has come to the hospital to visit his grandfather. Sentence 6 and sentence 1 share a similar meaning, indicating that the person mentioned is currently at the hospital. However, sentence 7 is not very common or grammatically conventional in standard English.