"I have to go buy a car."
Translation:Tengo que ir a comprar un automóvil.
There are certain prepositions that accompany verbs when using multiple infinitive verbs. If you remember the lesson a while back using "ir" as an auxiliary verb for future tense, every single combination was a conjugated form of "ir" + a + infinitive verb. The same is true for other linked verbs (tener +que. Correr +a. Dejar +de. Etc.) I will post a helpful link once I am able to get to my home computer so you can have a better understanding of how this works.