Tafsir of Al-A'la 87:6

God gives him the glad tidings of a clear sign: That Gabriel will recite the revelation to him, and though he is unlettered—neither reading nor writing—he will memorize it and not forget it.

{Except what God wills}: He removes it from his memory by lifting its ruling and its recitation, as in His saying: "Or We cause it to be forgotten" (Al-Baqarah: 106).

It is said: He used to hasten the recitation when Gabriel dictated it to him, so it was said: "Do not hasten, for Gabriel is commanded to recite it to you repeatedly until you memorize it; then you shall not forget it, except what God wills—that you remember it after having forgotten it."

Or it is said: "Except what God wills" refers to the small amount and the rarity. As it is narrated that he omitted a verse during his prayer, and Ubayy thought it had been abrogated. He asked him, and he replied: "I forgot it."

The purpose is to negate forgetting entirely. It is like a man saying to his companion: "You are my partner in all that I possess, except what God wills," not intending to exclude anything. It is the use of the "small amount" to signify negation.

It is also said: His saying {فلا تنسى} (so you shall not forget) is a prohibition, and the alif is added for the rhyme (fāṣilah), as in His saying: "the way" (Al-Aḥzāb: 67). It means: Do not be negligent in reciting and repeating it, lest you forget it, except for what God wills to make you forget by lifting its recitation for a benefit.

{Indeed, He knows what is manifest}: Meaning, you recite aloud along with Gabriel—peace be upon him—for fear of it slipping away. God knows your reciting aloud with him and what is in your soul that prompts you to do so. Do not do it, for I will suffice you against what you fear.

Or it means: He knows what you conceal and what you manifest of your words and deeds, what is apparent and hidden of your states, and what is beneficial or corrupting for your religion. Thus, He causes to be forgotten of the revelation what He wills, and leaves preserved what He wills.

{And We will ease you toward ease. So remind, if the reminder benefits. He who fears will be reminded. But the wretched one will avoid it—He who will burn in the greatest Fire, then he will neither die therein nor live.}