Learn Hardcore Korean: naeil chinguwa daehoee chamgahaeyo. - I will participate in the competition with a friend tomorrow.

In Korean, basic time expressions (like 오늘, 내일, 어제) can function as adverbs when placed at the start of a sentence. You usually drop the particle 에. Saying 내일에 친구와… isn’t wrong, but omitting 에 sounds more natural and conversational.

What does 친구와 mean, and why 와 instead of 과?

친구와 means “with a friend.” The particle 와/과 means “and” or “with.” You use 와 after a noun ending in a vowel (like 친구), and 과 after one ending in a consonant.

Could I use 친구하고 or 친구랑 instead of 친구와?

Yes. 친구하고 and 친구랑 also mean “with a friend.”

친구하고 is neutral spoken style.
친구랑 is more colloquial/familiar.
친구와 is slightly more formal or written.

Why is there no subject marker (이/가 or 은/는)?

Korean often omits explicit subjects when they’re clear from context. Here, the speaker (“I” or “we”) is understood, so you don’t need 저는 or 나는.

What function does 에 have in 대회에 참가해요?

The particle 에 marks the target or object of participation with the verb 참가하다 (to participate). So 대회에 참가해요 literally means “(I) participate in a competition.”

Why can’t I say 대회에서 참가해요?

에서 indicates the physical location where an action takes place (“at the venue”), but 참가하다 already carries the meaning “to join/enter.” You mark the event itself with -에. If you wanted to say “At the competition venue, (we) chat,” you’d use 경기장에서 이야기해요, but for “participate in the competition,” it’s 대회에 참가해요.

How do you get 참가해요 from the dictionary form 참가하다?

참가하다 is a 하다-verb. To make the polite present tense, drop 다 and add 해요. So:
참가하다 → 참가하 + 요 → 참가해요

Why is the present tense used for something happening tomorrow?

Korean often uses the present tense for scheduled or planned future events, especially if they’re fixed or certain. It’s like saying “The train leaves at 9,” using present tense for a future timetable.

Does 친구 here refer to one friend or could it mean “friends”?

Without the plural marker 들, 친구 usually means one friend. If you want to say “with friends,” you’d say 친구들과 참가해요 or 친구들이랑 참가해요.