Learn Hardcore Korean: jigeum gibuni johayo.

이 is the subject marker attached to nouns ending in a consonant. Here, it tells us that 기분 (“mood/feeling”) is the subject of the adjective 좋아요. If the noun ended in a vowel, you would use 가 instead.

Could I use 기분은 instead of 기분이 in this sentence?

Yes, but the nuance changes.

기분이 좋아요 simply states “(My) mood is good.”
기분은 좋아요 highlights or contrasts the topic “as for my mood, it’s good (but …).” You often see it paired with a contrasting clause:
- 기분은 좋아요, 그런데 일이 많아요. (“My mood is good, but I have a lot of work.”)

What’s the difference between 좋다 and 좋아하다?

좋다 is an intransitive descriptive verb (adjective) meaning “to be good” or “to feel good.” It takes its target as the subject:
- 기분이 좋다 → “(My) mood is good.”
좋아하다 is a transitive verb meaning “to like” someone or something. It takes its target as the object:
- 음악을 좋아하다 → “to like music.”
  In speech, you conjugate 좋다 as 좋아요 and 좋아하다 as 좋아해요 in polite style.

Where does the adverb 지금 fit in, and what does it do?

지금 means “now” or “right now.” It’s an adverb and usually appears before the main predicate (verb/adjective).

지금 기분이 좋아요.
You could also say 기분이 지금 좋아요, but fronting 지금 emphasizes the timing more strongly.

What is the function of -요 at the end of 좋아요?

The ending -요 makes the sentence polite (존댓말). Without it, you’re speaking informally (반말):

Polite: 지금 기분이 좋아요.
Casual: 지금 기분이 좋아.

Can I drop particles like 이 or even 지금 in casual speech?

Yes, in spoken casual speech Koreans often omit particles and adverbs when context is clear:

기분 좋아. (“I feel good.”)
Or just 좋아. if the topic is obvious.
But in writing or polite conversation, it’s better to keep the particles and adverbs.

How do I make the negative form: “I’m not feeling good right now”?

Insert 안 before 좋다:

지금 기분이 안 좋아요. (“I’m not feeling good right now.”)
You can also use 별로 for “not really/so-so”:
지금 기분이 별로예요.

How would I ask someone “Are you feeling good now?” using this structure?

Turn it into a question by raising intonation or adding a question mark in writing:

지금 기분이 좋아요?
In spoken polite style, you can also add …? with a slightly softer ending:
지금 기분이 좋아요?
In casual speech:
지금 기분이 좋아?