Learn Hardcore Korean: naeireun joheun narieyo.

은 is the topic marker in Korean. By attaching 은 to 내일 (tomorrow), the speaker sets “tomorrow” as the topic of the sentence—what they’re talking about. It highlights that, regarding tomorrow, “it is a good day.”

What’s the difference between 은/는 and 이/가, and why choose 은/는 here?

은/는 (topic markers) introduce or contrast a topic.
이/가 (subject markers) simply mark the grammatical subject, often for new information or focus.
In 내일은 좋은 날이에요, using 은 suggests that the speaker is talking about or contrasting “tomorrow” (e.g., compared to today or yesterday). If you said 내일이 좋은 날이에요, it would feel like you’re simply stating “Tomorrow is a good day” as brand-new information, without the nuance of contrast or topic.

What part of speech is 좋은, and how is it formed?

좋은 is the adjective 좋다 (“to be good”) turned into a modifier for a noun. You form it by taking the adjective stem 좋- and adding -은 (for descriptive verbs/adjectives ending in a consonant). So 좋다 + -은 = 좋은, meaning “good (_).” Here it modifies 날 (“day”).

What does 날 mean, and why not use another word for “day”?

날 means “day” in the general sense (daytime, calendar day). You could also say 하루, but 하루 often emphasizes the duration of one day. In the expression 좋은 날이에요, 날 is the standard, neutral word for “day” and fits the pattern adjective + noun.

Why is the ending -이에요 used, and what level of formality does it convey?

-이에요 is the polite declarative ending for nouns and noun phrases in the present tense. It corresponds roughly to English “is/am/are.” It’s suitable for everyday polite conversation. The full breakdown:
• 날 (day) + 이 (the copula stem for nouns ending in a consonant) + 에요 (polite ending) = 날이에요 (“(It) is a day.”)

Could I use 예요 instead of 이에요, like 좋은 날이에요 → 좋은 날예요?

No. Whether you use -이에요 or -예요 depends on the final sound of the noun before the copula:

If the noun ends in a consonant (날), use -이에요 → 날이에요.
If it ends in a vowel (학교), use -예요 → 학교예요.

Is it possible to drop 날 and just say 내일은 좋아요? Would that mean the same?

You can say 내일은 좋아요, but it sounds more like “Tomorrow is good” with an implied meaning (e.g., “Tomorrow is good for me”). Including 날 makes it explicit: “Tomorrow is a good day.” Without 날, the listener must infer what exactly about tomorrow is “good.”