1. He broke his legs, and he's in the hospital now.

2. He broke his legs, and he's in hospital now.

3. The boys are playing in the hospital.
4. The boys are playing in hospital.
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Which ones are correct? What is the difference between them?

Americans usually say "the" hospital.

People in the United Kingdom usually say in hospital -- without the article.

1. He broke his legs, and he's at the hospital now.

2. He broke his legs, and he's at hospital now.

3. The boys are playing at the hospital.
4. The boys are playing at hospital.
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I changed 'in' into 'at.'
Which ones are correct? What is the difference between them?

Americans use "the", people in the U.K. don't.

3 and 4 don't make a lot of sense since children don't usually play at a hospital.

something is wrong with my legs all my joints

Both forms are grammatically correct, but there is a difference in how they are used.

1. "He broke his legs, and he's in the hospital now."
2. "He broke his legs, and he's in hospital now."

In the first sentence, "the hospital" is preceded by the definite article "the." This indicates that the person is in a specific hospital, possibly one that has been mentioned before or is known to both the speaker and the listener.

In the second sentence, "hospital" is used without an article. This implies that the person is in a hospital in general, without specifying a particular one.

Regarding the second set of sentences:

3. "The boys are playing in the hospital."
4. "The boys are playing in hospital."

Similar to the first two sentences, the use of "the hospital" in the third sentence suggests a specific hospital, whereas "hospital" in the fourth sentence implies any hospital in general.

It's important to note that the usage of "the hospital" or "hospital" depends on the context and the specific intention of the speaker.