They can't just assume you were the one driving your car--the prosecutor must prove that you were actually the one driving or operating the motor vehicle.
You were pulled over for a valid reason.
The authorities can't just stop you for no reason. Part of what they must prove is that when you were pulled over, they had a reasonable belief that you were violating a law.
Everyone's favorite high school history term. The prosecution needs to prove that when they arrested you it was "more likely than not" that you were driving while intoxicated.
They will, in most cases, need to prove that at the time you drove the vehicle, your Blood Alcohol Concentration was .08 or above (or .02 and up if you are under 21). Refusing a chemical sobriety test after being placed under arrest for DWI in Virginia is a separate charge, so even if you think you will fail the test, you are still legally required to take it.