Tafsir of Ad-Dukhan 44:42

"Except those upon whom Allah has mercy..."

This is in the nominative case (raf') as a substitute (badal) for the pronoun in "they are helped" (yunsarun), or it is in the accusative case (nasb) as an exception (istithna') from it. That is to say: they shall not be prevented from the punishment, except for those whom Allah the Exalted has shown mercy, by pardoning them and accepting intercession on their behalf.

It has been permitted that it be a substitute or an exception from "protector" (mawla), and in this—as in the first view—is proof of the validity of intercession. However, the first [interpretation] is more probable both linguistically and in meaning. The exception, regardless of what it is drawn from, is connected (muttasil). Al-Kisa’i said: It is disconnected (munqati'), meaning: "But those whom Allah has shown mercy, for they are not in need of a relative to benefit them nor a helper to assist them." There is no merit to this view given the evident connectedness [of the exception]. Yes, [the argument for it being disconnected] does not apply if the exception is from the pronoun, as the pronoun refers to the disbelievers; so do not be heedless of this.

"Indeed, it is He who is the Exalted in Might" — the Overpowering, whom none can assist when He intends to punish someone.

"The Merciful" — to those whom He, the Almighty and Majestic, intends to show mercy.