Is it ever effective to use modern and archaic grammar together?

What year? I assume this is 100+ years ago. After you decide on the year, go find the books published that year in Guttenburg. Read those and absorb the manner of speech. More importantly, decide what your readers can put up with. I can't bear to read authentic archaic anything. "He said 'What's up?' in a quaint Kentish dialect." work great for me as a reader. On the otherhand, the thee's and thou's are just as much an intellectual speedbump as if someone replaced all the she's and he's with ze (one of the proposals for a gender neutral pronoun!) In any case, as a reader, the fact someone is saying hither and thither at all is what sounds wrong, not that they are failing to use it to mean "to here" and and "to there", or in the case of thee and thou, if they are using it in the nominative or accusative slot.