When you make a car payment (or almost any type of loan payment) the first thing that payment is applied to is the interest due with that payment. Next, if the payment is late then any late fees, fines or penalties are subtracted from the payment. Then what is left is applied to the principle. So if your car payment is $300 a month and you want to pay $400 you can most certainly do that. Doing so will knock down your principle an additional $100 for that month.
But No, you can not pay *ONLY* the principle. In some situations you can pay only the interest. But unless you've got some kind of weird loan agreement (which would not be provided by a legitimate lender) you can not pay *ONLY* the principle.
I've helped friends in situations where they could not make a mortgage payment one month. So we called the bank prior to the due date of the payment and they allowed payment of interest only for that month and just tacked an additional payment on the end of the 360 month loan. This allowed the borrower to get through a rough month (major car repairs) and still keep their credit and payment history intact.
"U don't know what you talking about."
Selling cars does not mean one understands how loans work. Loans can be set up many ways, and in a fair number of cases conditions of the loans can be altered when doing so is favorable to the lender first, and the borrower second. Additionally, personal attacks in this public forum are uncalled for and serve no purpose. That's what facebook is for.