Learn Hardcore Korean: i kibodeuneun soriga jagaseo bame sseodo gwaenchanhayo. - This keyboard is quiet, so it’s okay to use it at night.

Breakdown of i kibodeuneun soriga jagaseo bame sseodo gwaenchanhayo.

Why is 키보드 followed by 는 in 이 키보드는?

는 is the topic marker. In 이 키보드는, the speaker is introducing this keyboard as the topic and then saying something about it.

So the structure is basically:

이 키보드는 = as for this keyboard
소리가 작아서 밤에 써도 괜찮아요 = its sound is quiet/small, so it’s okay to use at night

Using 는 often gives a feeling like:

It is very natural here because the sentence is describing a characteristic of the keyboard.

Why is there also 가 in 소리가 작아서? Why not just use one subject marker?

This is a very common Korean pattern.

이 키보드는 sets the overall topic: this keyboard
소리가 marks the specific thing being described about that topic: its sound

So the sentence works like this:

Korean often does this:

Some similar patterns:

So 소리가 작다 is describing one property of the keyboard: its sound.

Does 작다 really mean small here? Why is it used for sound?