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 ? What does mean?