Один однокурсник сразу стал моим близким другом.

One course‑mate immediately became a close friend of mine.

Breakdown of Один однокурсник сразу стал моим близким другом.

• друг: the friend
• мой: my
• стать: to become
• один: one
• сразу: immediately
• близкий: close
• однокурсник: the course-mate

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?

Стать means to become, not to be.
стал моим близким другом = became my close friend (change of state).

• был моим близким другом = was my close friend (already true, no change implied)
• стал моим близким другом = he wasn’t my close friend before, but then he became one

The word сразу (immediately) fits very naturally with a change verb like стать.

Why is it моим близким другом and not мой близкий друг?

Because after стать (and also after быть, становиться) Russian uses the instrumental case for professions, social roles, and similar predicative nouns.

• Nominative: мой близкий друг
• Instrumental: моим близким другом

So the pattern is:

• Он стал врачом.
• Он стал моим другом.
• Этот город стал нашим домом.

In your sentence, all three words (моим, близким, другом) are in the instrumental singular masculine to match each other.

How can I recognize that моим близким другом is instrumental? What are the endings?

Masculine singular instrumental usually has:

• noun: -ом / -ем → другом
• adjective: -ым / -им → близким
• possessive мой in masc. sg. instr.: моим

So:

• друг → другом
• близкий друг → близким другом
• мой близкий друг → моим близким другом

Why is it стал (masculine) and not some neutral form? What if the classmate is female?

The verb in the past tense agrees with the gender of the subject.

• однокурсник is a masculine noun → past tense must be masculine: стал.

• If you were talking about a female classmate, you would say однокурсница (feminine noun), and the verb would be стала:

  • Одна однокурсница сразу стала моей близкой подругой.

What does сразу add here, and where can it go in the sentence?

Сразу means immediately / right away / at once.
Here it modifies стал, telling us how quickly the person became a close friend.

Default and most natural position: before the verb:

• Один однокурсник сразу стал моим близким другом.

You can move it a bit, but other positions sound less neutral and may change emphasis slightly:

• Один однокурсник стал сразу моим близким другом. (still okay, light emphasis on сразу)
• Сразу один однокурсник стал моим близким другом. (focuses on “right away, this one classmate…” – more marked, story-telling style)

Can I say сразу же instead of just сразу? Is there a difference?

Yes, you can say:

• Один однокурсник сразу же стал моим близким другом.

Сразу же is basically сразу + a slight emotional/emphatic boost, often translated the same: immediately / right away.
The difference is subtle: сразу же can sound a bit more expressive or insistent than plain сразу, but both are correct and natural.

Is один однокурсник literally “one classmate,” or does it sound like “a certain classmate”?

It can mean both, depending on context, but in storytelling it’s usually understood as:

• “one particular classmate / a certain classmate”

If the speaker wanted to emphasize quantity (“only one, not more”), they would usually make that clearer in context or add something:

• Только один однокурсник стал моим близким другом. = Only one classmate became my close friend.

Could I drop один and just say Однокурсник сразу стал моим близким другом?

Grammatically, yes, but the nuance changes.

• Один однокурсник… introduces one specific, previously unknown character into the narrative.
• Однокурсник сразу стал… sounds more like you already know which classmate, or you’re speaking generically (e.g., as a job description: “A classmate of mine quickly became my close friend”).

In a typical story-telling context, один однокурсник is more natural for introducing a new person.

Is the whole sentence stylistically neutral, or does it sound formal / informal?

The sentence is stylistically neutral.
You can use Один однокурсник сразу стал моим близким другом in:

• spoken conversation
• informal writing (messages, emails)
• neutral written texts (essays, narratives, memoirs)

It’s neither slangy nor overly formal.