Matthew 6:17 (King James Bible) but thou when thou fastest anoint thine head and wash thy face
As explained under 1 Nephi 11:2, the Book of Mormon scribes tended to accidentally use the inflectional ending -eth instead of -est for the second person singular pronoun thou. Here in the printer’s manuscript, Oliver Cowdery wrote “thou fasteth”, but the 1830 edition reads “thou fastest” (the same as in the corresponding Matthew 6:17). It is possible that 𝓞 itself read “thou fasteth”; even so, that would have still been an error for “thou fastest”.
We also note here that the King James text reads “thine head” while the Book of Mormon text reads “thy head”, which is what we expect since the h of head is pronounced in standard modern English. For further discussion of the competition between thy and thine in the Book of Mormon, see under 3 Nephi 12:43. For other cases where the Book of Mormon version of the Sermon on the Mount uses the more expected language of modern English, see the list under 3 Nephi 12:13.
Summary: Maintain fastest and thy in 3 Nephi 13:17; the occurrence of thou fasteth in 𝓟 represents one of the scribes’ typical errors in the Book of Mormon manuscripts (namely, the use of the inflectional ending -eth in place of -est); since the initial h in head is pronounced in standard modern English, the occurrence of thy rather than the King James thine is not surprising.