Get it on or get on with it.

Prepositions can get you in trouble. This especially likely to happen when you know you need to use one, but don't know exactly which one to use. You are forced to guess, which usually involves a crude translation of the prepositions of your native language. The results are unpredictable, and it is easy to embarrass yourself.

Take the three expressions: "Get on," "Get on with it," and "Get it on." All three are easily confused with each other by their words alone, but their meanings are radically different. I think the first is more common in British English. You would ask "How are you getting on with the new neighbors?" to ask about someone's general impression of the situation with the new neighbors. As in, "we're getting on well. They are nice people." "To get on" is basically equivalent to "how's it going," which I think is more common in American English. As in, "how's it going with the new neighbors?" "It's going well, they are nice people."

This is all part of boring small talk. But if you were to use another expression and say, "how are you getting it on with the neighbors," you would be asking a very different question: "how are you having sex with the neighbors?" because "Getting it on" is a euphemism for "to have sex." Saying this wouldn't be the end of the world, to be sure; people would probably know what you meant and the situation would probably resolve itself without trouble. But more than likely you want to look smart and be smooth; and, god forbid, if the person you are talking to is having sex with the neighbors you just might get more information than you wanted to have.

Misusing "to get on with it" is less of a problem. "Getting on with it" is simply proceeding to do or begin something that is being delayed for some reason. It is often an expression of impatience, or an expression that the thing to be done is unpleasant. A related expression is "to get it over with." For example: "after three hours in the waiting room at the dentist's office, I was thinking, 'jesus let's just get on with it!' If I'm going to get my tooth pulled, I just want to get it over with." These two expressions, "to get on with it" and "to get it over with" are very nearly interchangeable.

But not totally interchangeable. Bringing all these together in a sentence: "If you are getting on well with someone, and you want to get it on with them, you would probably like to just get on with it." However, you would not want to "get it over with." You want to "get on with" getting it started, not with concluding it. In general, you should not use either "get on with it" or "get it over with" in any situation that is even remotely amorous or romantic. You may be thinking it, but you probably won't get on well if you say it.