Learn Hardcore Korean: hangugeoneun yeongeoboda eoryeowoyo.

는 is the topic particle. It marks 한국어 (“Korean language”) as the topic of the sentence—the thing you’re talking about—so you could translate it loosely as “As for Korean…”

Why do we use 보다 after 영어?

보다 means “than” in comparisons. When you say 영어보다, you’re saying “than English.” It always attaches directly to the noun you’re comparing to.

Why don’t we need the word 더 (“more”) to say “more difficult”?

Because 보다 already indicates a comparison, adding 더 is optional or even redundant. You can say 영어보다 더 어려워요 (“It’s more difficult than English”), but 영어보다 어려워요 is perfectly natural.

Why is the sentence ending 어려워요 instead of 어렵다?

어렵다 is the dictionary (base) form of the adjective. 어려워요 is the polite present-tense conjugation you’d use in everyday conversation or with someone you don’t know well.

Could I use 이/가 instead of 은/는 in 한국어는 영어보다 어려워요?