Learn Hardcore Korean: naeil chinguwa yaksokhaeyo. - I make an appointment with a friend tomorrow.

In Korean, the present polite form (-해요) is often used for scheduled or planned future events when there’s a time word like 내일. So 약속해요 here can be understood as “I’ll meet” or “I have plans” tomorrow.

What exactly is 약속해요, and how is it formed?

약속 is a noun meaning promise, plan, or appointment.
하다 means to do.
When you combine them, you get the verb 약속하다 (“to make a plan/appointment” or “to promise”).
In polite present form, it becomes 약속해요.

What role does the particle 와 play in 친구와?

와 is the comitative particle meaning with when attached to a noun ending in a vowel. So 친구와 means with a friend. If the noun ended in a consonant, you’d use 과 instead.

Why is there no subject like 저는 in this sentence?

Korean often omits the subject if it’s clear from context. Here, “I” is implied, so adding 저는 is optional and usually dropped in everyday conversation.

What’s the difference between 와/과, 하고, and (이)랑 when saying “with”?

• 와/과: more formal or written.
• 하고: neutral, common in speech.
• (이)랑: casual, colloquial.
All three can mean with when indicating who you’re doing something with.

How would you say 내일 친구와 약속해요 in a more casual style?

You can use the casual ending -아/어/해 and switch 와 to 랑:
내일 친구랑 약속해.

Can 약속해요 also mean “I promise” rather than “I have plans”? How do you tell the difference?

Yes, 약속하다 can mean both “to promise” and “to make a plan/appointment.” In 친구와 약속해요, because you’re using the comitative particle 와 (“with friend”), it naturally means “I’m meeting” or “I have plans.” To express “I promise (to do something),” you’d typically specify the promise’s content or recipient, for example:

도와주기로 약속해요 (“I promise to help”)
너에게 약속해요 (“I promise you”).