what would make the car only go forward even if put in reverse?
You don't mention if this is an auto or manual trans, but I would say that whatever is going on, you have a problem internal to the transmission. A friend had a 1992 Nissan Maxima with manual trans that eventually would not go into reverse at all. Knowing the Maximas, the automatic transmission is electronically controlled. Have this diagnosed by a Nissan dealer.
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