Abinadi shifts the focus from himself to the priests. They have allowed him to do this because they have asked a question of scriptural interpretation, and they must be able to defend their interpretation. Abinadi asks what they teach. They answer that they teach the law of Moses. Their answer requires some explanation.
I have suggested that the significant difference in the Noah rule over that of his father, Zeniff, is that Noah has adopted aspects of a foreign culture and religion. While this incursion of foreign elements is the source for their "whoredoms" and "idolatry," it could not have come at the complete abandonment of all of the culture/religion of the people prior to that time. Change rarely works this way (even under the duress of outright conquest the conquered peoples still attempt to retain some of their previous culture and religion). Thus the incursion of new ideas into the society was overlaid on the foundation of the Mosaic law, and doubtless justified through it. It is entirely conceivable that the multiple wives was justified by Solomon and David, just as occurred during Jacob's time.
The conceptual foundation of the people was the law of Moses, even though it had been interpreted to allow the entrance of the elements that Mormon found distasteful, and against which Abinadi is directly preaching. Nevertheless, we should not presume that the power of the idea of living the law of Moses was diminished. Thus when Abinadi asks what is preached, the quick answer is the law of Moses. In the minds of the priests this should put Abinadi at the disadvantage. What they are attempting to do is place themselves as defenders of the law of Moses, and by contrast, Abinadi would have to be against the law of Moses, and therefore culpable.
Of course, the tactic does not work, and Abinadi turns up the pressure on the priests of Noah.
Variant: The 1830 text for verse 27 reads "…therefore what teachest thou this people.." (emphasis added). This has been changed to "therefore what teach ye this people…" (emphasis added. Book of Mormon Critical Text. FARMS 1987, p. 428).
The shift from "thou" to "ye" is needed grammatically because Abinadi is addressing a group of people, and "thou" is the singular form. There is no significant meaning lost through the change, and in fact, the modern usage must reflect the underlying text better than the 1830's version. The fact is that a native speaker would not make this type of mistake, but Joseph could easily make the error since this form of English was not native to him. As with other similar types of errors in the Book of Mormon text, the best suggestion is that the translation method allowed for some participation of Joseph's language, while remaining faithful to the general meaning of the underlying text.