"It's quiet in the hospital."
I wouldn't say that 很 should necessarily be translated as 'very' in English. I'd say it'd be best to translate it as 'is/am/are' most of the time as the degree of emphasis is nowhere near as high as very in English. It has some degree of emphasis, but you'd rarely say 這個貴. It sounds almost incomplete, so 很 is often used as a copula of sorts which has some emphasis, but not a whole lot of it. Someone correct me if I'm wrong, but you can exaggerate and draw out the tone of 很 to indicate that something is VERY expensive/good/loud etc. Otherwise if you need more emphasis, you can use 真，太<sub>了，</sub>極了，超級，非常 or even 好 which is a good alternative for very.
I feel that Duo quite often uses 很 in Chinese that it's not translating into the English model answers as "very". Is it normal to use 很 so much more in Chinese than English, and how should the idea of "very" be expressed in English if 很 doesn't do this?
Yes, 很 is required in some cases in Chinese grammar, and it's essentially playing the role of "is". Unless it's being stressed, 很 won't really get translated as "very".
Yes, in fact it's necessary to use 很 (or another adverb of degree like 真 or 非常) in sentences like these, where the adjective is in place of the verb. In such cases, the meaning of 很 is bleached to nothing more than a placeholder. To achieve an actual intensifying like with English "very", you can either emphasise 很 or use a stronger adverb like 非常.