To cheat vs. to cheat on

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WRONG: My friend Tina cheats her boyfriend quite often.
RIGHT: My friend Tina cheats on her boyfriend quite often.

Cheating on your partner is wrong, and you know what’s also wrong? Not using the word ‘on’ in the sentence! If you say, „Tina cheats her boyfriend“, it suggests that she cheats him out of something, like money. She tricks him into taking his money.

If you wish to say that she is unfaithful, however, you must say, „Tina cheats on her boyfriend.“

Another form of cheating is during exams, when students look at answers they’ve hidden on tiny pieces of paper. Here, the preposition changes, and you need to use the word ‘in’. So, in a full sentence – „Honza cheated in his mathematics exam.“

So, once again, you cheat ‘on’ a person, you cheat ‘in’ a test, and you cheat someone (no preposition) out of money.