Learn Hardcore Korean: chingudeuri noraereul deutgo isseoyo.

In Korean, -들 is an optional plural marker attached to nouns. It simply indicates “more than one” (so 친구들 = “friends”). However, Korean often relies on context rather than explicitly marking plurals. If the number is clear from context, you can leave out -들 entirely.

Why is -이 used after 친구들 in 친구들이? Could I use -가 or -는 instead?

-이/가 are subject-marking particles. After a consonant you use -이, after a vowel -가. By contrast, -는/은 are topic markers.
• 친구들이 marks “friends” as the grammatical subject.
• 친구들은 would mark “friends” as the topic (“As for friends…”), shifting the emphasis slightly.
• 친구가 (singular subject) could also be used if you wanted “a friend” instead of “friends.”

What does -를 do in 노래를? Can it be omitted?

-를 is the object particle (used after a vowel) that indicates 노래 (“song”) is the direct object of 듣다 (“to listen”). In casual speech you might drop it if context is crystal clear, but in writing or careful speech you almost always include -를 to avoid ambiguity.

How do you form the present continuous 듣고 있어요 from 듣다? What does -고 있어요 mean?

To express “be …-ing,” Korean uses verb stem + -고 있다.

Start with 듣다 (to listen).
Remove -다, add -고 있다 → 듣고 있다 (“is listening”).
Make it polite present → 듣고 있어요.
So -고 있어요 is the progressive aspect, equivalent to English “is/am/are …ing.”

Why is it 듣고 and not 들고? I thought 듣다 sometimes became 들다.

듣다 is an irregular ㄷ verb that only changes ㄷ → ㄹ when the following ending begins with a vowel (e.g. 듣 + 어요 → 들어요). The suffix -고 begins with ㄱ (a consonant), so no irregular change happens and the stem stays 듣 → 듣고.

What is the role of 있어요 here? Is it part of the main verb or just an auxiliary?

Here 있어요 is an auxiliary verb paired with -고 to form the progressive aspect (-고 있다 = “to be …ing”). The main verb is still 듣다 (“to listen”); 있어요 simply indicates that the action is ongoing.

What politeness level is the ending -어요? How would you say this sentence more formally or casually?

-어요 is the plain polite form (suitable for most everyday situations).

Formal polite: 친구들이 노래를 듣고 있습니다. (ending -습니다)
Casual/informal: 친구들이 노래 듣고 있어. (drop 요 and particle 를 if very casual)

Can I change the word order, like saying 노래를 친구들이 듣고 있어요?

Yes. Korean has flexible word order because particles mark each word’s role. 노래를 친구들이 듣고 있어요 is grammatically correct, but the standard Subject-Object-Verb order (친구들이 노래를 듣고 있어요) is most natural in everyday speech.

What’s the difference between 친구들이 노래를 들어요 and 친구들이 노래를 듣고 있어요?

들어요 (simple present) can mean a habitual or general action: “Friends listen to songs” (they do this regularly).
듣고 있어요 (present continuous) means the action is happening right now: “Friends are listening to a song.”

Can I drop 친구들이 and simply say 노래를 듣고 있어요? When is it OK to omit the subject in Korean?

Yes—if the subject is clear from context, you can omit it. Korean often leaves out pronouns and subjects when both speaker and listener know who or what is meant. 노래를 듣고 있어요 could mean “I’m listening to a song,” “(They’re) listening to a song,” etc., depending on context.