Tafsir of Yusuf 12:66

{He said, "I will never send him with you..."} This contradicts my current state, for I have seen from you what I have seen regarding sending him with you.

{...until you give me a solemn promise by Allah} Until you give me that by which I may be assured, from Allah. He intended for them to swear to him by Allah. He made swearing by Allah a "solemn promise" from Him because swearing by Him is that by which covenants are confirmed and strengthened. Allah has permitted this, so it is a permission from Him.

{...that you will surely bring him back to me} This is the response to the oath, for the meaning is: "Until you swear that you will surely bring him back to me."

{...unless you are surrounded} Unless you are overcome and thus unable to bring him, or unless you are destroyed.

If you ask: "Inform me of the reality of this exception, for there is a difficulty in it?" I say: {That you are surrounded} is a maf'ul lahu (causative object). The affirmative statement, which is his saying {that you will surely bring him back to me}, is interpreted as a negation. Its meaning is: "You shall not refrain from bringing him except for being surrounded." That is: You shall not refrain from it for any reason whatsoever, except for one reason: that you are surrounded. It is an exception from the most general of the general in the causative object, and an exception from the most general of the general only occurs in negation. Therefore, it must be interpreted as a negation. Its parallel in affirmative speech interpreted as negation is their saying: "I swear by Allah, la-ma (I will) do it, and illa (otherwise) I will do it," meaning: "I ask nothing of you but the action."

{...over what we say} Regarding the request for the solemn promise and the giving of it.

{...is a Guardian} A watcher, one who is aware.

{And he said, "O my sons, do not enter from one gate, but enter from different gates; and I cannot avail you against Allah at all. The decision is only for Allah; upon Him I have relied, and upon Him let the reliant rely." And when they entered from where their father had ordered them, it did not avail them against Allah at all, except for a need in the soul of Jacob which he fulfilled. And indeed, he was a possessor of knowledge because We had taught him, but most of the people do not know.}