co tu wyjaśniać, tak sie mowi po prostu :-)
sprawdz sobie w słowniku kolokacji
ja mam 'Oxford Collocations' i tm pod hasłem 'hospital' jest
at a/the hospital -> She works at the John Radcliffe Hospital
in (a/the) -> He is in hospital recovering from ...
to (at/a) -> He's been taken to hospital for ...
" we can be in or at a building. in means inside only ; t could mean inside or in the grounds or just outside"