중 (中)

This is a good one to learn. If you've never taken an interest in 한자 (Hanja) before, make this one your first. It's a doosy and it's everywhere. There are a few 중 out there but this one is the more common one known as:

가운데 중 = 中
가운데, 안, 속, 사이, 진행, 마음, 맞다, 곧다, 바르다, 뚫다, 고르다
It essentially means the middle of center of something (among other meanings)

Common hanja examples:
중국 = 中國 = middle country = China
중학생 = 中學生 = middle learning life = junior high student
중 = 中 = (medium sized (i.e. meal)

대 = 大 = (big, large-sized (i.e. meal)

What might perplex the most is its substitution for the ~고있다 verb ending. To emphasize or simply express in a shorter way that something is happening now (present progressive) then instead of using ~고있다 use ~는중이다.

일하는 중이에요 I'm working
가는 중이야 I'm going
공부하는 중이야 I'm studying
책을 읽는 중이에요 I'm in the middle of reading a book.
통화 중이에요 The line is busy.
수업 중에 자지마세요 Please don't sleep during class.
버스를 기다리는 중이에요 I'm waiting for the bus.
나 영어 공부 열심히 하는 중 I'm diligently studying English
CCTV녹화중 You are being recording on closed circuit television right now.