I put "The car cost nothing" which was marked as wrong. This basically means past tense as in the "The car did not cost anything" as opposed to "The car costs nothing" which is present tense. I accept that Duolingo means the present tense here but how does one distinguish in German and, indeed, what is the commoner phrase? - past ("cost") or present ("costs")?
present tense: das Auto kostet nichts
past tense: das Auto kostete nichts or das Auto hat nichts gekostet
Both are common; it depends on whether you are speaking about a car that is in front of you and that you are about to "buy"/"sell" (present tense) or that you "bought"/"sold" in the past.