(There is no blame on the Prophet in that which Allah has made legal for him.) means, in that which has been permitted for him and which he has been commanded to do, i.e. his marrying Zaynab, may Allah be pleased with her,, who had been divorced by his adopted son Zayd bin Harithah,
(That has been Allah's way with those who have passed away of old.) means this is the ruling of Allah for the Prophets who came before him. Allah would not command them to do anything for which they might be blamed. This is a refutation of those hypocrites who imagined that there was anything wrong with his marrying the ex-wife of Zayd, his freed slave and adopted son.
(And the command of Allah is a decree determined.) means, His command which He has decreed must inevitably come to pass; nothing can prevent it or avert it, for whatever He wills happens, and whatever He does not decree, does not happen.