"Maybe that is not beer."
Translation:Misschien is dat geen bier.
In Dutch, if the subject (here: "dat") is not the first word of the sentence, then the subject and the verb switch places. This is known as inversion, there is more information about it here.
This word order is not strict. "Misschien" is an adverb, which can appear at many places in the sentence. Moving it to the beginning of the sentence gives it more emphasis. Also good would be: "Dat is misschien geen bier" (That is maybe not beer).