Learn Hardcore Korean: jigeum deowoseo eeokeoneul kyeoyo. - It's hot right now, so I turn on the air conditioner.

It’s the connective -아서/어서, which links two clauses with a cause-and-result meaning: “because/since/so.” In this sentence, it means “because it’s hot (now), [I] turn on the AC.” It’s showing the reason for the second clause.

Why is it 더워서 and not 덥어서?

The adjective 덥다 (to be hot, weather/ambient) is a ㅂ-irregular. When a vowel-initial ending like -아서/어서 is attached, the ㅂ changes to 우, and then contracts:

덥다 → 더우 + 어서 → 더워서 Related forms:
Present polite: 더워요
Past: 더웠어요
Conditional: 더우면
Reason with -(으)니까: 더우니까

Can I use 뜨거워서 instead of 더워서?

Generally no.

덥다 is used for weather/air or how a person feels due to ambient heat (It’s hot/the room is hot/I’m hot).
뜨겁다 is for things that are hot to the touch (hot water, hot plate) or body temperature in a medical sense (e.g., 몸이 뜨거워요 for “I have a fever”). So “Because it’s hot now” is naturally 지금 더워서, not 지금 뜨거워서.

Why is it 에어컨을 with 을? Could it be 에어컨이?

The verb 켜다 (to turn on) takes a direct object, so the object marker 을/를 is used: 에어컨을 켜다 (“turn on the AC”).
Using 에어컨이 would make the AC the subject, which fits different verbs like state expressions: 에어컨이 켜져 있어요 (“The AC is on.”).

Can I drop the object particle and say 에어컨 켜요?

Yes. In conversation, object and topic particles are often omitted when context is clear. 에어컨 켜요 is very natural in speech.

Should I say 켜다 or 틀다 for “turn on the AC”?

켜다 is the standard verb for switching on lights/devices: 에어컨을 켜다.
틀다 originally means “twist/turn,” but colloquially people say 에어컨을 틀다 and it’s widely understood. In careful or formal usage, 켜다 is preferred.
To turn off: 끄다 (e.g., 에어컨을 끄다).

What tense/politeness is 켜요? How do I say other tenses?

켜요 is present polite (해요체). It often covers “I turn on / I’m going to turn on (now).”

Past: 켰어요
Future/plan: 켤 거예요
Suggestion: 켤까요? (“Shall we turn it on?”)
Volunteering/promise: 켤게요
Negative: 안 켜요 / 켜지 않아요 More formal style: 켭니다; very casual: 켜.

There’s no subject—who is turning it on?

Korean often drops obvious subjects. Here, context usually implies “I” (or “we”). If you need to be explicit:

Topic: 저는 지금 더워서 에어컨을 켜요.
Subject focus: 제가 에어컨을 켤게요. (“I’ll turn it on.”)

Can I use -(으)니까 instead of -아서/어서? Any nuance?

Yes: 지금 더우니까 에어컨을 켜요 also works.
Nuance:

-아서/어서 often sounds like a natural/obvious cause.
-(으)니까 can feel a bit more like giving a justification, and it’s the usual choice when the main clause is a command or suggestion (see next Q).

Is 더워서 에어컨을 켜세요 okay?

Prescriptive grammar typically avoids -아서/어서 when the main clause is a command/suggestion. Prefer -(으)니까:

Better: 더우니까 에어컨을 켜세요. For plain statements (not commands), -아서/어서 is fine: 더워서 에어컨을 켜요.

Where can I put 지금? What about 지금은?

지금 더워서 에어컨을 켜요 (as given) emphasizes the current heat as the reason.
지금 에어컨을 켜요 emphasizes the timing of the action (“I’m turning it on now”).
에어컨을 지금 켜요 also focuses on “now.”
지금은 marks a contrast (“now, as opposed to other times”): 지금은 더워서 에어컨을 켜요 (“Now it’s hot, so…”).

How is the sentence pronounced naturally?

지금 [지금]
더워서 [더워서]
에어컨을 is pronounced with liaison as [에어커늘] (the final ㄴ of 컨 moves to the following vowel).
켜요 [켜요] Together: roughly [지금 더워서 에어커늘 켜요].

Does 켜고 있어요 mean “I’m turning it on right now”?

Not usually. 켜다 is momentary, so 켜고 있어요 tends to mean “It’s on (and staying on)” in many contexts. To say you’re in the act of turning it on:

지금 켜는 중이에요. For “It’s on (state)”: 에어컨이 켜져 있어요.
For immediate intention: 지금 켜요 or 지금 켤게요 (I’ll turn it on now).

What’s the difference between 지금 and 이제 here?

지금 = “now (at this moment).”
이제 = “now, from this point on / already,” often signaling a change of state.
In this sentence, 지금 is the natural choice. 이제 더워서 에어컨을 켜요 would mean “It’s hot (from now on/these days), so we (now) use the AC,” which has a different nuance.