Learn Hardcore Korean: hoeuiga kkeutnago naseo dongryowa iyagihaeyo. - After the meeting ends, I talk with a colleague.

The suffix -고 나서 combines the connective ending -고 (“and/then”) with 나서 (from 나다, “to occur/arise”) to express a clear sequence of actions: “after doing X, do Y.”

끝나다 → “to end”
끝나고 나서 → “after it ends”
So in 회의가 끝나고 나서 동료와 이야기해요, it means “I talk with my colleague after the meeting ends.”
You can contrast:
• 끝나고 이야기해요 – “(Once it) ends, I talk…” (simple sequence)
• 끝나고 나서 이야기해요 – “after it ends, then I talk…” (more explicit)

Why is 회의가 marked with -가 instead of -는?

-가 is the subject marker, used to introduce or spotlight the doer/action in a neutral or new-information context. -는 is the topic marker, suggesting known information or drawing contrast.
Here, 회의가 simply marks “meeting” as the subject of 끝나다 without implying any contrast or background topic. If you said 회의는, you’d be making “the meeting” a general topic and possibly implying additional contrast (e.g. “as for the meeting, …”).

What does 동료와 mean, and why use -와 instead of -를 or another particle?

동료와 means “with a colleague.”

와/과 (or colloquially -하고) indicates companionship or “together with.”
-를/을 marks a direct object; 동료를 이야기해요 would mean “I talk about a colleague,” not “talk with.”
You could also say 동료하고 이야기해요 (more casual) or 동료와 함께 이야기해요 (slightly more emphatic “together with”).

Why isn’t 동료 marked with -를, as in 동료를 이야기해요?

Because 이야기하다 in the sense “to have a conversation with someone” treats the person as a conversational partner, not a direct object. Instead of -를, you use
• 와/과 or 하고 for “with”
• 에게/한테 for “to someone”
If you did attach -를, it would change the meaning to “talk about a colleague.”

What’s the difference between 이야기해요 and 말해요?

Both relate to speaking, but:

말하다 = “to say” or “to speak” (often one-way; “I say something”)
이야기하다 = “to talk,” “to have a conversation” (implies back-and-forth exchange)
Use 이야기해요 when you want to express chatting or discussing with someone.

What tense and politeness level does 이야기해요 convey?

This is the -아/어요 polite informal speech level (해요체), non-past tense. It can express habitual actions (“I talk”), general facts, or present/future events in polite everyday conversation.

Can I replace 끝나고 나서 with 끝난 후에, and are there any nuances?

Yes. 끝난 후에 (using -은 후에) also means “after it ends.”

끝나고 나서 feels more conversational and explicit.
끝난 후에 is slightly more formal or written.
Both are grammatically correct:
• 회의가 끝나고 나서 동료와 이야기해요.
• 회의가 끝난 후에 동료와 이야기해요.

Is it okay to drop 나서 and say 회의가 끝나고 동료와 이야기해요?

Yes. -고 alone can mark sequence (“once/after X happens”). Saying 회의가 끝나고 동료와 이야기해요 is grammatically fine and common. Adding 나서 just makes the sequence more explicit and emphatic.