"The police have stopped a car."
Translation:Politiet har stoppet en bil.
Because of the "har" we used the past participle. This is different from the past tense in pretty much all Danish verbs (I can't think of any exceptions off the top of my head). For verbs that follow the standard rules it goes as follows:
For verbs whose simple past tense adds an "-ede" the past participle adds an "-et" instead
For verbs whose simple past tense adds "-te", the past participle adds "-t"
The difference is like in English simple past for "to see" is "saw" (så") and the past participle is "seen" (set) where the first acts alone when talking about events in the past, where the second is used after perfect tenses (to have done something) and in the passive voice (to be done)