English is a language bursting with ambiguity and double meanings. But the words "on" and "off" would have to be two of the worst offenders. I was thinking about words that foreign-language speakers would surely find particularly hard to master, when learning to speak English. And I couldn't go past these two. From the most basic meaning of the words, which relates to position — e.g. "the book is on the table", and "the plane is off the ground" — "on" and "off" have been overloaded more thoroughly than an Indian freight train.
To start with, let's focus on the more common and important meanings of these words. The most fundamental meaning of "on" and "off", is to describe something as being situated (or not situated) atop something else. E.g: "the dog is on the mat", and "the box is off the carpet". "On" can also describe something as being stuck to or hanging from something else. E.g: "my tattoo is on my shoulder", "the painting is on the wall". (To describe the reverse of this, it's best to simply say "not on", as saying "off" would imply that the painting has fallen off — but let's not go there just yet!).
"On" and "off" also have the fundamental meaning of describing something as being activated (or de-activated). This can be in regard to electrical objects, e.g: "the light is on / off", or simply "it's on / off". It can also be in regard to events, e.g: "your favourite TV show is on now". For the verb form of activating / de-activating something, simply use the expressions: "turn it on / off".
But from these simple beginnings… my, oh my, how much more there is to learn! Let's dive into some expressions that make use of "on" and "off".
Bored this weekend? Maybe you should ask your mates: "what's on?" Maybe you're thinking about going to Fiji next summer — if so: "it's on the cards". And when you finally do get over there, let your folks know: "I'm off!". And make sure your boss has given you: "the week off". If you're interested in Nigerian folk music, you might be keeping it: "on your radar". After 10 years spreading the word about your taxidermy business, everyone finally knows about it: "you're on the map". And hey, your services are "on par" with any other stuffed animal enterprise around.
Or we could get a bit saucier with our expressions. Next time you chance to see a hottie at yer local, let her know: "you turn me on". Or if she just doesn't do it for you: "she turns me off" (don't say it to her face). Regarding those sky-blue eyes, or that unsightly zit, respectively: "what a turn-on / what a turn-off". After a few drinks, maybe you'll pluck up the courage to announce: "let's get it on". And later on, in the bedroom — who knows? You may even have occasion to comment: "that gets me off".
And the list "goes on". When you're about to face the music, you tell the crew: "we're on". When it's you're shout, tell your mates: "drinks are on me". When you've had a bad day at work, you might want to whinge to someone, and: "get it off your chest". When you call auntie Daisy for her birthday, she'll probably start: "crapping on and on". When you smell the milk in the fridge, you'll know whether or not it's: "gone off".
Don't believe the cops when they tell you that your information is strictly: "off the record". And don't let them know that you're "off your face" on illicit substances, either. They don't take kindly to folks who are: "high on crack". So try and keep the conversation: "off-topic". No need for everything in life to stay: "on track". In the old days, of course, if you got up to any naughty business like that, it was: "off with your head!".
If you're into soccer, you'll want to "kickoff" to get the game started. But be careful you don't stray: "offside". If you can't "get a handle on" those basics, you might be "better off" playing something else. Like croquet. Or joining a Bob Sinclair tribute band, and singing: "World, Hold On".
That's about all the examples I can think of for now. I'm sure there are more, though. Feel free to drop a comment with your additional uses of "on" and "off", exposed once and for all as two words in the English language that "get around" more than most.