Learn Hardcore Korean: naneun deo noryeokhal tenikka oneureun bandeusi jesigane wa, seodureuji ma. - I’ll try harder, so please come on time today; don’t rush.

-(으)ㄹ 테니까 combines -(으)ㄹ 테다 (speaker’s strong intention/expectation) + -니까 (since/because). It means “since I fully intend/expect to…”.
With a 1st-person subject like 나는, it signals the speaker’s commitment and sets up a request/command in the next clause.
Compared to:
- -(으)ㄹ게(요): a promise reacting to the listener; by itself it doesn’t license a following command. You’d say … 할게, 그러니까 제시간에 와.
- -(으)니까 alone: just a reason, without the “my firm intention” nuance.

Why is 오늘은 using the topic marker 은? What changes if it’s just 오늘?

오늘은 is contrastive: as for today/at least today (implying today is special—maybe you were late before).
오늘 (no particle) simply means “today” as a time adverb, with no contrast.
Both are correct; 오늘은 matches the likely emphasis here.

Can I flip the order to 오늘은 반드시 제시간에 와, 나는 더 노력할 테니까?

It’s grammatical but sounds less natural and a bit like an afterthought.
With -니까, the reason normally comes first, then the instruction: … 할 테니까, … 와.
If you want command-first, make two independent sentences without -니까: 오늘은 반드시 제시간에 와. 나는 더 노력할게.

Why is it 와 and not 오?

From 오다
- the plain imperative -아, you get vowel contraction: 오 + 아 → 와. So the casual imperative is 와 (come).
Other useful forms:
- Polite imperative: 오세요.
- Blunt/plain imperative: 와라 (can sound bossy).
- 와요 is usually indicative or a soft suggestion depending on intonation; the standard polite request is 오세요.

What register is this? How would I say it politely?

Does 반드시 differ from 꼭?

What exactly does 제시간에 mean? Are 정시에 or 제때 okay?

제시간에 = at the proper time → on time (appointments, meetings).
Alternatives:
- 제때(에): at the right time (very common, a bit looser).
- 정시에: at the exact scheduled time (clock-precise, formal).
- 시간 맞춰(서)/맞게: in time/as scheduled.
Use -에 with 제시간 because it marks a point in time; 제시간으로 is not used here.

Is it fine to link 와 and 서두르지 마 with a comma?

Isn’t “be on time” and “don’t rush” contradictory?

The nuance is: plan ahead and make it on time, but don’t rush recklessly right now.
Korean often pairs a strong request with a safety caveat. You can make it explicit: 오늘은 반드시 제시간에 와. 그렇지만 서두르지는 마.

Do I need 나는? Could I drop it or say 난?

You can omit it: 더 노력할 테니까 … is fine when the subject is clear from context.
나는 adds emphasis/contrast (“as for me”).
난 is the common spoken contraction of 나는.

Why is it 서두르지 and not 서둘지 or 서둘러지?

Base verb: 서두르다. With the negative imperative -지 마, attach -지 directly to the stem: 서두르- + 지 마 → 서두르지 마.
The 르-irregular shows only before vowel-initial endings (e.g., 서둘러요). -지 begins with a consonant, so the stem stays 서두르-.
서둘지 마 / 서둘러지 마 are incorrect here.

How do 서두르지 마, 서두르지 마세요, and 서두르지 마라 differ?

Can -(으)ㄹ 테니까 take non–first-person subjects?

Yes. Then it conveys the speaker’s strong expectation/supposition about them: 걔도 곧 올 테니까 기다리자.
With 1st person, it mainly signals intention or promise.

Where can I place 반드시? Is 제시간에 반드시 와 okay?

Adverbs are flexible. All of these are natural, with slight shifts in focus:
- 오늘은 반드시 제시간에 와.
- 오늘은 제시간에 반드시 와.
- 반드시 오늘은 제시간에 와. (extra stress on “without fail” applying to today)

Should it be 오다 or 가다 here?

오다/와 = come (toward the speaker or the agreed meeting point conceived as the speaker’s side).
가다/가 = go (away from the speaker).
For asking someone to meet you or come to where you’ll be, use 오다/와; use 가다/가 when sending them somewhere else.

Any spacing/orthography gotchas like 할 테니까 or 하지 마?

Write a space in 할 테니까 / 할 테니 / 할 텐데 (not 할테니까/할텐데).
Negative imperatives also take a space: 하지 마 / 서두르지 마 (not 하지마/서두르지마 in standard writing).