Learn Hardcore Korean: naneun jeonhyeo pigonhaji anha.

Using 내가 (subject marker 가) instead would emphasize “I” as the specific doer/holder of the state: 내가 전혀 피곤하지 않아 (“It’s I who isn’t tired”).
In everyday speech, the topic often gets dropped if it’s obvious: 전혀 피곤하지 않아.
In polite speech, switch 나 to the humble 저: 저는 전혀 피곤하지 않아요.

Can I drop 나는?

Yes. Korean frequently omits the subject when it’s clear from context. 전혀 피곤하지 않아. is perfectly natural. You can also use the spoken contraction 난 for 나는: 난 전혀 피곤하지 않아.

Why is the negative formed as 피곤하지 않아 instead of just using 안?

Korean has two common negatives:

Adverb 안 before the predicate: 안 피곤해
Auxiliary -지 않다 after the stem: 피곤하지 않아
Both are correct. 안 feels a bit more casual; -지 않다 is slightly more neutral/formal or explicit. With “at all,” both sound fine: 전혀 안 피곤해 = 전혀 피곤하지 않아.

What exactly does 전혀 mean, and does it require a negative?

전혀 means “completely/entirely (not)” and almost always pairs with a negative to give “not at all.”

Natural: 전혀 피곤하지 않아, 전혀 안 피곤해, 전혀 몰라요, 전혀 없어
Don’t use 전혀 with a positive predicate unless you’re doing special rhetoric; it will sound odd.

Where should 전혀 go in the sentence?

Put 전혀 right before the negative predicate or before 안:

전혀 피곤하지 않아.
전혀 안 피곤해.
Avoid splitting it oddly (e.g., 피곤하지 전혀 않아)—that’s unnatural.

How do I make this polite or more formal?

Polite casual: 저는 전혀 피곤하지 않아요. / 전혀 안 피곤해요.
Formal: 저는 전혀 피곤하지 않습니다.
Plain/dictionary: 전혀 피곤하지 않다.

Is the spacing 피곤하지 않아 correct? Why is there a space?

Yes. In -지 않다 constructions, the auxiliary verb 않다 is written as a separate word: …지 않다 / …지 않아요 / …지 않았어요. So: 피곤하지 않아 is correct spacing.

How is this pronounced?

전혀 is pronounced roughly like “jeon-hyeo.”
피곤하지 like “pi-gon-ha-ji.”
In 않아, the ㅎ often drops; it sounds like 아나. So 피곤하지 않아 sounds close to “pi-gon-ha-ji a-na.”

Can I say 못 피곤해?

No. 못 expresses inability with action verbs (e.g., 전혀 못 자요 = “I can’t sleep at all”). 피곤하다 is a descriptive verb (an adjective), so you negate it with 안 or -지 않다, not 못.

Are there alternatives to 전혀 with different strengths?

Yes, all typically pair with a negative:

하나도 (도) / 조금도: “not even one/bit,” very strong: 하나도 안 피곤해, 조금도 피곤하지 않아요
별로: “not particularly,” weaker: 별로 피곤하지 않아
전혀: “not at all,” strong: 전혀 피곤하지 않아

Can I change the tense?

Yes:

Past: 전혀 피곤하지 않았어. / …않았어요.
Future/intention: 전혀 피곤하지 않을 거야. / …않을 거예요.
Progressive isn’t used with 피곤하다; just use the plain forms above.

Is 피곤하다 the only way to say “tired”?

피곤하다: “to be tired/fatigued” (neutral, everyday).
힘들다: “to be tough/hard; to feel worn out” (broader hardship, not just fatigue). E.g., 오늘 정말 힘들었어 can imply you’re exhausted.
피로하다: more formal/medical “fatigued.”
In your sentence, 피곤하다 is the natural choice.

Is there a shorter or older form like 피곤치 않아?

Yes, -지 않다 sometimes contracts to -치 않다 in writing or older/literary style: 피곤치 않아. It’s understood but feels a bit old-fashioned; 피곤하지 않아 is safer in modern speech.

Does using 나는 add any nuance?

Using 나는 can add contrastive emphasis, like “Me, I’m not tired at all (even if others might be).” Without it, 전혀 피곤하지 않아 is a simple statement about yourself in context.