Learn Hardcore Korean: chingureul wihae chaegeul sayo.

위해 is a noun form of the verb 위하다, meaning “to do something for the benefit of someone or something.” When combined with 친구를, the phrase 친구를 위해 literally means “for the sake of a friend” or “for a friend.” It indicates the purpose or beneficiary of the action.

Why is 친구 marked with -를 here and not with -에게?

The particle -를 marks 친구 as the direct object of the verb 위하다 (“to do for”). 친구를 위해 = “for a friend.” In contrast, -에게 marks an indirect object or recipient in a giving action (e.g., 친구에게 선물해요 = “give a gift to a friend”). Since 사다 is simply buying for someone’s benefit rather than directly giving, we use -를 with 위해.

Can I use 위해서 instead of 위해? Is there a difference?

Yes—친구를 위해서 책을 사요 is also perfectly natural. 위해 and 위해서 both mean “for” in this context, with no real change in meaning. 위해 is slightly more common in formal writing, while 위해서 appears more often in spoken, casual Korean. They’re interchangeable here.

How is 사요 formed from the dictionary form 사다?

The verb 사다 (to buy) has the stem 사. Since the last vowel is ㅏ, you add -아요. The combination 사 + 아요 becomes 사아요, which contracts to 사요 (the extra 아 is dropped in speech). So 사요 is simply the polite present-tense form of 사다.

What tense and politeness level is 사요?

사요 is in the present tense and uses the -요 ending, making it polite (but not overly formal). This yo-form is the standard polite register used in everyday conversations with acquaintances, strangers, and in many semi-formal situations.

Why is there no subject (like “I”) in 친구를 위해 책을 사요?

Korean frequently omits the subject when it’s understood from context. Here, the speaker (“I”) is implied. If you needed clarity or emphasis, you could say 제가 친구를 위해 책을 사요, but in most situations the subject isn’t stated.

Can you swap 친구를 위해 and 책을, like 책을 친구를 위해 사요?

While Korean word order is relatively flexible, the most natural sequence is:
[Adverbial/Purpose phrase] + [Object] + [Verb]
→ 친구를 위해 책을 사요
You could say 책을 친구를 위해 사요, and it’s still correct, but it sounds less idiomatic in everyday speech. Stick with the original order for the smoothest flow.