Learn Hardcore Korean: oneureun nalssiga malgaseo changmuneul yeoreosseoyo. - Today the weather was clear, so I opened the window.

The particle -은 marks 오늘 (“today”) as the topic of the sentence. It signals that what follows is information about today. Without -은, you could still say 오늘 날씨가 맑아서…, but using 오늘은 places extra emphasis or contrast on “today” (e.g. “as for today…”).

Why is 날씨 followed by -가 instead of -은?

Here, -가 marks 날씨 (“weather”) as the subject of the subordinate clause 날씨가 맑아서. Subject particles -이/가 introduce new information or specify what is doing/being something. If you used 날씨는, you’d be treating “weather” as a broader topic or making a contrast, which changes the nuance.

What does the connective -아서 in 맑아서 indicate?

The ending -아서 (or -어서) connects two clauses to show cause or reason. 맑아서 means “because it’s clear,” indicating that the clear weather is the reason for the action in the main clause.

Why do we say 맑아서 and not 맑고 here?

-고 simply links clauses like “and,” giving a neutral sequence or addition. -아서, however, specifically expresses causation. In this sentence, you need a causal link (“because it was clear”) rather than just “and.”

Why is the verb 열었어요 in the past tense? Could it be in the present tense?

The suffix -었어요 marks past tense, so 열었어요 means “opened.” If you wanted to express present tense (“(I) open” or “(I’m) opening”), you’d use 열어요 instead.

What is the function of -을 in 창문을 열었어요?

The particle -을 marks 창문 (“window”) as the direct object of the transitive verb 열다 (“to open”). It tells you what was opened.

How could you say this sentence more casually or more formally?

Casual:
오늘은 날씨가 맑아서 창문을 열었어.

More formal:
오늘은 날씨가 맑아서 창문을 열었습니다.