Learn Hardcore Korean: oneul nalssiga deowoyo.

오늘 means “today” and here it functions as a time adverb, so it doesn’t need a subject or topic particle. It simply sets the timeframe for the sentence.

Why do we use 가 after 날씨 instead of 는?

가 marks 날씨 as the subject being described (new or focused information). If you used 는 (“오늘 날씨는 더워요”), you’d be making the weather the topic, as in “As for the weather today, it’s hot.” Both are grammatically correct but carry slightly different nuances.

What is 덥다, and how does it become 더워요?

덥다 is the dictionary form meaning “to be hot.” It’s a ㅂ-irregular adjective, so when conjugating to the polite present tense you drop the ㅂ and add 워요, yielding 더워요.

Can we say 덥어요 instead of 더워요?

No. Because 덥다 is an irregular ㅂ-adjective, the correct polite form is 더워요, not 덥어요.

Why is the adjective at the end of the sentence?