Один однокурсник сразу стал моим близким другом.

the friend

Breakdown of Один однокурсник сразу стал моим близким другом.

• мой: my
• стать: to become
• один: one

Questions & Answers about Один однокурсник сразу стал моим близким другом.

Why do we use один here? Does it just mean the number one, or is it more like a / some in English?

In this sentence один is closer to a / one particular than to just the number one.
It suggests:

• not just any classmate, but one specific classmate from the group
• often with a nuance like “there was this one classmate who…”

Russian has no article a, so speakers sometimes use один to introduce a new, specific person into the story: Один мужчина подошёл ко мне… = One man / A man came up to me…

What exactly does однокурсник mean, and how is it different from одноклассник?

Однокурсник = someone who studies on the same course / year at a university or other higher-education institution.

• один + курс → однокурсник: literally “same-course person”
• Used mostly for university / college peers, same year / same program

Одноклассник is for school classmates (same class/form at school).
So:

• at school: одноклассник
• at university: однокурсник

Why is the verb стал used instead of something like был?