Learn German: Das Auto, das da steht, gehört dem Kommissar. - The car that is standing there belongs to the detective.

The first Das is the regular definite article ("the") for Auto. The second das is a relative pronoun, meaning "that" or "which". It refers back to the neuter noun das Auto, and because it acts as the subject of the mini-sentence inside the commas, it takes the nominative case.

Why does the verb steht come right before the second comma?

The phrase , das da steht, is a relative clause. It gives extra information about the car. In German, a relative clause always pushes its conjugated verb to the very end of that clause, right before the closing comma.

Why is it dem Kommissar instead of der Kommissar?

The verb gehören (to belong to) always forces the owner into the dative case. Since Kommissar is a masculine noun (der Kommissar), it changes to dem Kommissar in the dative.

The translation says "is standing", but the German sentence just uses steht. Is there no "-ing" form?

Exactly! German doesn't have an equivalent to the English continuous "-ing" tense. The simple present tense steht translates to both "stands" and "is standing" depending on what sounds best in English.