I often hear UK folk saying "in hospital," whereas we in the USA say, "in the hospital," or "in a hospital." Why does real ( ) English use an understood article, whereas US English says it?

Americans almost never say "on a vacation", they say "on vacation". Of course, they never say "on trip", they say "on a trip". And they never use "holiday" to describe vacation.

And they never use "holiday" to describe vacation.

A bit of cross-threading here, but I recently did refer to a vacation day as a "holiday," though I assumed I had been jaded by our British friends, here.

I will hear "on vacation" and "on a vacation" about evenly, though you are right that I would never hear "on trip."

Along those lines, I rarely hear "journey" used here, as in "on your train journey," though I heard it all the time in England.

and vs German....
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A little crossthreading here.

One of the things I tell me elementary students is not to worry too much if they get articles wrong as in almost every case the meaning will still be clear and attempting to lay down rules for use of indefinate/definite/zero article constructions is very difficult if not actually impossible.

Anyway in the Schadenfreude thread there's been some question of German use of the article. jheem has explained it over there but I've got a bit to add. It often seems to people that German uses articles much more than English does but I think this just comes from the way that in German, as in French, when a noun is quoted on its own out of context the article is always included. (For example in answer to the question (Wie sagt man "horse" auf Deutsch?, How do you say "horse" in German?. das Pferd as "horse" is a neuter noun.)

Otherwise usage is generally similiar to English but note this from the book German Grammar by Elke F. Gschossmann-Hendershot:

(I've stripped out the long lists of examples and test questions.)

SPECIAL USES OF THE DEFINITE ARTICLE

With General or abstract nouns. (This would include Schadenfreude)

With the names of streets, lakes, mountains and countries (except with the names of countries which are neuter unless the name of the coumtry is modified with an adjective)

With weights, measures and expressions of time

With parts of the body or articles of clothing.

OMISSION OF THE INDEFINITE ARTICLE

Before a Predicate Nominative
(I'll explain that one :
It's when we say "He is a dentist." In German the "a" is omitted giving the sentence "Er ist Zahnarzt."
With certain set phrases, for example "Er hat Fieber"/"He has A fever."
After the preposition "als" meaning "as a(n)".

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BAck to English now.

With regard to "in hospital"/"in the hospital" they carry for me (here in England) different shades of meaning. "In the hospital" means physically inside the building whereas "in hospital" means sick and hospitalised.
How would US English distinguish these two meanings?