In the measure that We reveal4
this Qur'an unto thee, [0 Prophet,] We explain it to thee in the best possible way,5
seeing that ere this thou wert indeed among those who are unaware [of what revelation is].6
Or: "By Our having revealed".
Lit., "with the best explanation (ahsan al-iqtisas)". This rendering is very close to the interpretation given by Zamakhshari: "We set forth this Qur'an unto thee in the best way in which it could be set forth." According to Razi, it may safely be assumed that the adjective "best" refers not to the contents of "that which is set forth" - i.e., the particular story narrated in this surah - but rather to the manner in which the Qur'an (or this particular surah) is set forth: and herein he agrees with Zamakhshari. It should be borne in mind that the verb qassa (the infinitive nouns of which are qasas and iqtisas) signifies, primarily, "he followed step by step" or "by degrees", and, subsequently, "he related [a piece of news or a story] as though he followed its traces": hence, "he expounded [it] gradually" or "he explained [it]" (cf. Lane VII, 2526, quoting the Qamus and the Taj al-'Arus with specific reference to the above verse). If, on the other hand, the infinitive noun qasas is regarded as synonymous, in this context, with qis,sah ("story" or "narrative"), the above sentence might be rendered as "We narrate unto thee the best of narratives", i.e., the subsequent story of Joseph. In my opinion, however, the rendering "We explain it [i.e., the Qur'an] in the best possible way" is preferable inasmuch as it fully coincides with the two opening verses of this surah, which state, in effect, that the Qur'an is self-explanatory.
At this point in his commentary, Raz' draws the reader's attention to 42:52 "thou didst not know what revelation is, nor what faith [implies]": a passage similar in purport to the closing words of the above verse: hence my addition, between brackets, of the phrase "of what revelation is".