Tafsir of Al-Muddathir 74:8

“So when the naqur is sounded”—that is, when the trumpet is blown. Al-Naqur is derived from al-naqr, meaning to produce a sound; its origin is striking, which is the cause of the sound. From this comes the minqar (beak) of a bird, because it strikes with it. Due to this causal relationship, the term is used metaphorically for the sound itself, and this usage became common. It is intended here to mean the blowing [into the trumpet], as it is a type of striking.

The fa (in fa-idha) is for causality; it is as if it were said: "Endure their harm, for before them lies a terrifying Day in which they will encounter the consequence of their harm, and you will encounter the consequence of your patience regarding it." The operative element (‘amil) for idha is what is indicated by His saying: [The remaining verses of the passage].