Translation:He had a serious accident and is now in the hospital.
Duolingo is telling me I "need" the definite article "the" in sentence. I'm a native English speaker. The article is most emphatically optional here.
The article is most emphatically optional here --in the UK yes. But, I think it would be required in the US.
"In hospital" is also correct in Canada, but US influence is creeping in here, too.