Tafsir of Ta-Ha 20:116

(And when We said to the angels, "Prostrate to Adam") This is the beginning of the explanation of the aforementioned event and the manner in which his forgetfulness and lack of resolve appeared. (And when) is in the accusative case as an object for an implied verb addressed to the Prophet (may Allah bless him and grant him peace); that is, "And remember the time when We said to the angels," etc. It has been said: It is conjoined to an omitted phrase, meaning: "Remember this, and remember when We said," or it is a conjunction of one narrative to another. In either case, the intent is: "Remember what occurred at that time between Us and him, so that his forgetfulness and lack of resolve may become clear to you."

(So they prostrated, except Iblis) The discussion regarding this has passed repeatedly.

(He refused) This is an inaugural sentence that serves as an answer to a question arising from the report of his failure to prostrate, as if it were said: "What is the matter with him that he did not prostrate?" So say: (He refused). Al-iba' (refusal) is to refrain or the intensity of refraining. Its object is either omitted—meaning he refused the prostration, as in His, the Exalted’s, saying: "He refused to be among those who prostrate"—or it is not intended at all, by treating the verb as intransitive, meaning he performed the act of refusal and manifested it.