You are welcome to answer questions posted in the Ask a Teacher forum as long as your suggestions, help, and advice reflect a good understanding of the English language. If you are not a teacher, you will need to state that clearly at the top of your post. Please note, all posts are moderated by our in-house language experts, so make sure your suggestions, help, and advice house the kind of information an international language teacher would offer. If not, and your posts do not contribute to the topic in a positive way, they will be subject to deletion.
They both sound correct, but they have different meanings.
John cheated Mary.
John used some kind of dishonesty in order to take something away from Mary. Example: Mary gave John money to invest for her, but John spent the money on himself.
John cheated on Mary.
This sentence has only one meaning: John has promised Mary that he will not have romantic relations with any other women, but he broke that promise.
John cheated/cheated on Mary. 
Re: John cheated/cheated on Mary. 
Linear Mode
