Tafsir of Al-Muddathir 74:8

"So" (Fa) in His saying: {So when the trumpet is blown} is for causation. It is as if He said: "Be patient with their harm, for before them is a difficult day in which they will encounter the consequence of their harm, and you will encounter the consequence of your patience with it."

"So" (Fa) in {Then that} is for the consequence (Jaza').

If you ask: What governs the accusative case of idha (when), and how is it valid for yawma'idhin (that day) to be an adverbial modifier for yawmun 'asir (a difficult day)?

I say: Idha is governed by what the consequence implies, for the meaning is: "So when the trumpet is blown, the matter becomes difficult for the disbelievers." As for what permits yawma'idhin to be an adverbial modifier for yawmun 'asir, it is that the meaning is: "That is the time of the blowing, the occurrence of a difficult day," because the Day of Resurrection arrives and occurs when the trumpet is blown. There is disagreement over whether this is the first blast or the second.

It is also permissible for yawma'idhin to be indeclinable in the place of a nominative, as a substitute (badal) for dhalika (that), and 'asir (difficult) is the predicate, as if it were said: "The day of the blowing is a difficult day."

If you ask: What is the benefit of His saying {not easy} when 'asir (difficult) suffices for it?

I say: When He said {upon the disbelievers}, restricting the difficulty to them, He said {not easy} to indicate that it will not be for them as it is for the believers—easy and light. This is to combine the threatening of the disbelievers and the increase of their rage with the glad tidings and consolation of the believers. It is also possible that it means it is a difficulty from which there is no hope of it becoming easy, unlike the difficulties of worldly affairs which one hopes will become easy.