"The girl is standing beside a red car."

Translation: A lány egy piros autó mellett áll.

[deactivated user]

Why isn't it "A lanj egy piros autot mellett all." ???

[Findil]

Only the direct object gets the -t suffix, and there's no direct object in this sentence.

Most postpositions do not require a specific case, the noun preceding them will stay in the nominative form. "Mellett" (and "felett", "között", "alatt", etc.) belong to this group, so "autó mellett" is the correct phrase.