Learn Hardcore Korean: gamgi ttaemune ipmasi eobseo. - Because of a cold, I have no appetite.

때문에 means because of / due to and attaches directly to a noun: N + 때문에.
So 감기 때문에 = because of a cold.
With verbs/adjectives, use -기 때문에: e.g., 아프기 때문에 (because [I] am sick).

Why isn’t there an “I” in the sentence?

Korean often omits obvious subjects. Context usually makes “I” clear. If you want to include it:

Why is it 입맛이 and not 입맛은?

With 있다/없다, the thing that exists/doesn’t exist typically takes 이/가: 입맛이 없어.
입맛은 없어 adds contrast or topic emphasis: “As for appetite, I don’t have any (but other things might be fine).”

Is 없어 casual? How do I make this polite or formal?

Can I say 안 있어 to mean “don’t have”?

Use 없다 for “to not have / to not exist.” Don’t say 안 있어 for this meaning.

Good: 입맛이 없어.
Not natural: 입맛이 안 있어. Note: 안 있다 is fine for location (“not be [at a place]”), e.g., 집에 안 있어 (“I’m not at home”).

Is “감기가 때문에” correct?

No. Say 감기 때문에. 때문에 attaches straight to the noun; don’t add 이/가 or 은/는 before it.

Could I use -아서/어서 or -(으)니까 instead of 때문에?

Yes, with slight nuance differences:

감기라서 입맛이 없어. (since it’s a cold → natural, softer)
감기에 걸려서 입맛이 없어. (since I caught a cold)
감기니까 입맛이 없어. (because it’s a cold → more directly causal/justifying)
감기 때문에 입맛이 없어. (because of a cold → a bit more objective/neutral, common in writing)

Can I change the word order, like “입맛이 감기 때문에 없어”?

Put the reason phrase before the clause:

How do I pronounce 입맛이 없어 naturally?

Due to assimilation:

입맛이 → roughly 임마시 (im-ma-shi)
없어 → roughly 업써 (eop-sseo) So the whole sounds like: 감기 때문에 임마시 업써. (Romanization: gamgi ttaemune ipmasi eopseo)

What’s the nuance difference among 입맛, 식욕, and 밥맛?

입맛: everyday “appetite/taste” (most common in conversation).
식욕: “appetite” in a more medical/clinical sense (e.g., at a hospital).
밥맛: literally appetite for meals; 밥맛 없다 can also idiomatically mean “off-putting/annoying.” In a health context, it still means “no appetite.”

How can I say explicitly “I caught a cold” in this sentence?

Use 감기에 걸리다 (“to catch a cold”):

감기에 걸려서 입맛이 없어요. Avoid 감기가 있어요 for “I have a cold”; that’s not how Korean expresses it.

How would I say this about someone else politely (e.g., an elderly person)?

Use honorifics on the verb:

Are there natural intensifiers or softeners I can add?

Does this describe a general habit or a current state?

As-is, it’s a current state. To show timeframe: