Learn Hardcore Korean: mogyoireneun jeomsimsigane gyosireseo chingudeulgwa iyagihaeyo. - On Thursday, I chat with friends in the classroom during lunch.

목요일에 marks the time “on Thursday” neutrally.
Adding -는 as in 목요일에는 makes “Thursday” the topic or contrast point (“As for Thursdays, …”).
It emphasizes that this action happens regularly on Thursdays (maybe unlike other days).

Why are there two time/location markers (-에 in 점심시간에 and -에서 in 교실에서)?

점심시간에 uses -에 to mark when the action takes place: “at lunchtime.”
교실에서 uses -에서 to mark where the action takes place: “in the classroom.”
Korean separates time (-에) and action-location (-에서).

Could we use 교실에 이야기해요 instead of 교실에서 이야기해요?

교실에서 이야기해요 is correct for “talking in the classroom” (action happens there).
교실에 이야기해요 would mean “talk to (about) the classroom” or “talk at/to the classroom,” which sounds odd.
Use -에서 for location of an action.

Why is it 친구들과 이야기해요 instead of 친구들과 말해요?

Both 말하다 and 이야기하다 mean “to speak” or “to talk,” but:
• 말하다 often refers to the act of saying something (one-way speech).
• 이야기하다 implies a back-and-forth conversation or chatting.
Talking with friends suggests a conversation, so 이야기해요 is more natural.

What’s the difference between 친구들과, 친구들하고, and 친구들이랑?

All three mean “with friends,” but differ in style:

What does the ending -해요 signify? Could we say something else?

-해요 is the polite present tense ending in the 요-style.
Alternatives:
• 이야기한다: plain/formal style (often in writing or narration).
• 이야기해: casual (used with close friends, dropping 요).
• 이야기합니다: very formal (respectful, business contexts).

Can I reorder the phrases (time, place, comitative) in the sentence?

Korean is flexible with adverbial phrases, but common order is: time → place → comitative → verb.
You could say 목요일에는 교실에서 점심시간에 친구들과 이야기해요, but it sounds less natural.
Keeping related elements together (e.g., 점심시간에 교실에서) makes the sentence flow better.