1. My grandfather was in hospital.
2. My grandfather was in the hospital.
3. My grandfather was hospitalized.
(What does #1 and #2 mean? Are both #1 and #2 the same? Does #1 or #2 mean #3? He may have visited a patient. Don't you think so?)
#1 is the British English way to state this idea.
#2 is the American English way to state this idea.
#3 means the same as both of the first two.
(If someone were visiting in a hospital, the sentence would be this: My grandfather was at the hospital.)
Both sentences #1 and #2 carry similar meaning, but they can have slight differences in usage depending on the region.
In American English, sentence #2 is more commonly used, where "the hospital" refers to a specific medical facility. This sentence suggests that your grandfather was admitted to a hospital for medical treatment.
In British English, both sentences #1 and #2 can be used interchangeably, where "hospital" is used as a general term for any medical facility. However, sentence #2 is still more commonly used.
Sentence #3, "My grandfather was hospitalized," generally means that your grandfather was admitted to the hospital for medical treatment. It implies the same idea as sentences #1 and #2, but it is a more formal way of expressing it.
While it is possible that visiting a patient might be a reason for someone to be at the hospital, sentences #1, #2, and #3 specifically suggest that the grandfather was receiving medical care as a patient himself.