在
Meaning
- to exist; to be alive
- (of sb or sth) to be (located) at
- (used before a verb to indicate an action in progress)
How it's used
Where something is — 我在家 — and, before a verb, that it's happening right now: 我在吃饭. The second use is the ordinary way to say "-ing"; 正在 is the same thing with emphasis on this very moment.
Example sentence
不必在意。
Forget it.