Tafsir of Ta-Ha 20:37

"And indeed, We conferred favor upon you another time."

This is an initiation [of speech] intended to confirm what preceded it and to further settle the soul of Musa, peace be upon him, toward acceptance. This is achieved by clarifying that since the Almighty bestowed upon him those perfect favors without any prior supplication or request from him, then for Him to bestow upon him the like of them while he is a seeker and a supplicant is more appropriate and worthy. Its introduction with the oath [the particle la] is to show the utmost importance given to this matter.

That is: "And by Allah, We have conferred favor [another time]," meaning at a time other than this time. This is on the basis that ukhra (another) is the feminine of ukhar, meaning "different."

Marrah (time/instance) is an adverb of time, and what is intended by it is the extended duration in which occurred that which shall be mentioned—if Allah Almighty wills—regarding the great and numerous favors. It is originally a noun for a single passing, then it was applied to every single act, whether transitive or intransitive. It then became common for every individual unit of things that have recurring instances, until it became a hallmark for that, to the point that it was made a standard for other things of its meaning; so it is said, "This is the bina’ al-marrah (form denoting a single instance)." Similar to it are al-karrah, al-tallah, and al-duf’ah.

Abu Hayyan said: "What is intended by it is a favor other than this one, and ukhra is not the feminine of ula (first) with a kasra on the kha’ [in the sense of 'the latter'], such that it would be a counterpoint to the 'first'. Some have imagined that, saying: 'The Almighty named it ukhra (another), while it is ula (first), because it is ukhra (latter) in mention.'"