One way to say you are leaving is to say you are 'off'.
- I'm off now. See you tomorrow.
- It's time I was off or I'll be late for the meeting.
If you have no more supply/stock of something you are 'out of' the item.
- We're out of ink for the photocopier. Can you go out and get some?
- I'm afraid you are out of luck. I sold the last one an hour ago.
If you are depressed and miserable, you are 'down'.
- He's been very down since he lost his job.
- Why are you so down today?
When you have to submit something by a certain date, it has to be 'in' by then.
- The application has to be in before Friday.
- The report was supposed to be in last week but I'm still working on it.
If something is 'on', it is happening at the moment.
- There's a good play on at the theatre. (or should that be theater?)
- I'm going to London because the sales are on.
If somebody is not present, they are 'away'.
- I'm afraid he's away on holiday.
- I'm going to be away for a few days.
If food is no longer fresh, it is 'off'.
- I think this milk is off. Smell it.
- That melon is off. Don't eat it.
If you know some inside information, often a secret, you are 'in on' the information.
- Is Sarah in on our plans? Has anybody told her yet?
- He won't let me in on the secret.
If someone is 'up to' something, they are doing something secretive that you are not supposed to know about.
- I don't know exactly what is happening but Tom is up to something.
- Why all the whispering? What are you up to?
If someone is 'on' something, they are taking something as a form of drug.
- He's on 50 cigarettes a day at the moment.
- She's on her third cup of coffee and it's only 9 o'clock.