Learn Hardcore Korean: oneureun doseogwaneseo haru jongil gongbuhaesseoyo.

Breakdown of oneureun doseogwaneseo haru jongil gongbuhaesseoyo.

Why is 오늘 written as 오늘은 here?

The particle 은 marks 오늘 as the topic of the sentence.

This often gives a slight sense of contrast or sets the scene for what follows. In natural English, it may not always need to be translated directly.

So:

In this sentence, 오늘은 helps frame the whole statement around today.

Why is it 도서관에서 and not 도서관에?

Because 에서 marks the place where an action happens.

By contrast, 에 is often used for:

A very common beginner distinction is:

So:

What does mean exactly?