Learn Finnish: Urheilija työnsi autoa kilometrin. - The athlete pushed the car for a kilometer.

In Finnish, exact measurements of distance, time, weight, and price behave syntactically like total objects. Because this is an affirmative sentence with a regular subject (Urheilija), the measure phrase takes the genitive-shaped accusative (kilometrin), exactly as a normal total object would.

Why does this sentence have a partitive object (autoa) right next to an accusative measure phrase (kilometrin)?

They serve two different grammatical roles! Autoa is the direct object of the verb työnsi, and it is in the partitive because "pushing" is an ongoing, continuous action. Kilometrin is an adverbial measure phrase answering "how far?", and it follows the total object case rules independently of the main object.

If the sentence were negative ("The athlete didn't push the car for a kilometer"), would kilometrin change?

Yes, it would change to the partitive kilometriä. Because measure phrases follow the exact same case rules as direct objects, making the sentence negative forces the measure phrase into the partitive case: Urheilija ei työntänyt autoa kilometriä.

Could we use the genitive accusative for the car here (työnsi auton) to show the action was completed?