Learn Hardcore Vietnamese: Lan muốn gặp bác sĩ ở bệnh viện lớn. - Lan wants to see a doctor at the big hospital.

muốn means “to want.” In Vietnamese, when you express desire you use muốn + verb phrase. Here muốn gặp literally means “want (to) meet.” The structure is:
Subject + muốn + Verb (+ Object).

Why is gặp used here to mean “meet”? What’s the difference between gặp and thăm?

gặp = “to meet” or “to see (someone).” It emphasizes the act of encountering or meeting someone.
thăm = “to visit,” usually with a nuance of spending time with or checking in on someone (e.g., visiting a sick friend: thăm bạn ốm).
In your sentence, Lan isn’t “visiting” the doctor socially—she specifically wants to see or meet the doctor—so gặp bác sĩ is the natural choice.

Why isn’t there an article before bác sĩ, and how would I say “a doctor” or “the doctor” in Vietnamese?

Vietnamese has no direct equivalents of English articles (a, the). Nouns stand alone.

To say “a doctor,” you can add một (meaning “one”): một bác sĩ.
To say “the doctor,” Vietnamese often uses context or demonstratives: bác sĩ đó (“that doctor”) or vị bác sĩ (a polite classifier).
In your sentence, bác sĩ is indefinite and generic, so no article or classifier is needed.

What role does ở play in ở bệnh viện lớn, and how does it correspond to English prepositions?