gramatická analýza: Viděl rychle jedoucí auto.
Present active participle from jet (3pl. jedou plus -cí)
The main verb (the conjugated verb) is viděl; the subject
of this sentence is therefore on; the direct object is auto;
the PrAP that modifies the auto therefore is in the accusative neuter
Try paraphrasing the sentence as "He saw the car which was driving fast."