Learn Hardcore Korean: jumare chingudeulgwa noraebangeseo noraereul bulleoyo. - On weekends I sing at karaoke with friends.

-에 on a time word marks “on/at/in (time).” So 주말에 = “on the weekend.”
-에서 is for the place where an action happens, not for time, so you can’t use it with 주말.
For contrast/emphasis on the time, you can say 주말에는 (“as for the weekend, …”).

Why is there no subject? Who is doing the action?

Korean often drops the subject when it’s obvious. In everyday conversation, it usually means “I/we.” You can add it if you want:

저는 주말에 친구들과… (topic: “As for me, …”)
제가 주말에 친구들과… (subject focus: “I (as opposed to others) …”)

What exactly does 친구들과 mean? Do I need 들 to make “friends” plural? Could I say 친구와?

친구들 = “friends” (plural marker -들). It’s optional in many contexts, but it makes the plurality explicit.
-과/와 can mean “and/with,” so 친구들과 = “with friends.”
친구와 usually reads as “with a friend” (singular), though Korean sometimes leaves number to context.
In casual speech, even 친구랑 can imply multiple friends if the context makes it clear.

What’s the difference between 과/와, 하고, and (이)랑 here?

All can mean “and/with”; the difference is tone/register:

과/와: more formal/written. Use 와 after a vowel, 과 after a consonant. Here: 친구들 ends in ㄹ (a consonant) → 친구들과.
하고: neutral and very common: 친구들하고.
(이)랑: casual. 이랑 after a consonant, 랑 after a vowel: 친구들이랑.

Why is it 노래방에서 and not 노래방에?

-에서 marks the location where an action happens: 노래방에서 “at/in the karaoke (doing something).”
-에 is for destination or existence: 노래방에 가요 “(I) go to the karaoke,” 노래방에 있어요 “(I) am at the karaoke.” Since singing is an action, 에서 is correct here.

Do I need to include 노래를? Could I just say 노래방에서 불러요 or use 노래해요?

부르다 means “to sing” but also “to call.” Without an object, 불러요 can be ambiguous. 노래를 불러요 clearly means “sing (a song).”
If the context already makes it clear, you can drop the object: 노래방에서 (노래를) 불러요.
노래하다/노래해요 also means “to sing.” Many speakers prefer the set phrase 노래(를) 부르다 in everyday talk, but 노래해요 is fine too.

Why is it 불러요 and not 부러요? How does 부르다 conjugate?

부르다 is an 르‑irregular verb. With endings starting with 아/어, you: 1) drop ㅡ, 2) add an extra ㄹ to the preceding syllable, 3) then add -아/어(요). So: 부르- + -어요 → 불러요. Common forms:

Present polite: 불러요
Past: 불렀어요
Future: 부를 거예요
If/when: 부르면
Progressive: 부르고 있어요

How should I pronounce tricky parts like 불러요 and 친구들과?

불러요: double ㄹ pronounced as a clear long “l”: [bul-leo-yo].
친구들과: [chin-gu-deul-gwa] (the 과 is “gwa,” not “kwa”).
노래방에서: [no-rae-bang-e-seo] (the ㅅ in 에서 is “s,” not “sh”).
주말에: [ju-mal-e] (the ㄹ links forward, sounding like “ju‑ma‑re”).

Does 불러요 mean I sing now, usually, or in the future?

Korean present (-어요) covers present and habitual, and sometimes near future by context. Here it most naturally means a habit. To be explicit:

Habitual: 주말마다 노래방에서 노래를 불러요.
Right now: 지금 노래를 부르고 있어요.
Future plan: 노래를 부를 거예요.

Can I change the word order?

Yes. Keep the verb last and particles attached. For example:

친구들과 주말에 노래방에서 노래를 불러요.
노래방에서 주말에 친구들과 노래를 불러요. A common flow is Time → Companion → Place → Object → Verb, but you can reorder for emphasis.

How do I say “every weekend” or “usually on weekends”?

Every weekend: 주말마다 (e.g., 주말마다 친구들과…)
Usually/typically on weekends: 보통 주말에… / 주로 주말에…
“On weekends (in general)” in a contrastive sense: 주말에는…

Can I add 같이 (“together”), and where does it go?

Yes. Common placements:

주말에 친구들과 같이 노래방에서 노래를 불러요.
주말에 친구들과 노래방에서 같이 노래를 불러요.
Short: 같이 노래를 불러요.

What about spacing—do particles attach or get their own word?

Particles attach to the preceding word with no space:

주말+에 → 주말에
친구들+과 → 친구들과
노래방+에서 → 노래방에서
노래+를 → 노래를

How do I make this more formal or more casual?

Casual: 주말에 친구들이랑 노래방에서 노래를 불러.
Polite (your original style): …불러요.
Formal polite: …부릅니다. Also choose particles to match tone: 과/와 (formal), 하고 (neutral), (이)랑 (casual).

Can I say 친구들은 instead of 친구들과?

That changes the meaning. 친구들은 makes “friends” the topic/subject (“As for (my) friends, they…”), not “with friends.” To say “with,” use -와/과, -하고, or -(이)랑, optionally with 같이.

Is “노래방에 가서 노래를 불러요” a good alternative?

Yes. 노래방에 가서 노래를 불러요 explicitly says “(I) go to the karaoke and sing.” Your original sentence focuses on the singing at that location without mentioning the going.

Why is it 노래를 (with 를) and not 노래을?

Use 를 after a vowel-ending noun and 을 after a consonant-ending noun:

노래를 (vowel ending)
책을, 밥을 (consonant endings)
친구를 (vowel ending)