Surah Al-Hijr 15:96

ٱلَّذِينَ يَجْعَلُونَ مَعَ ٱللَّهِ إِلَـٰهًا ءَاخَرَ ۚ فَسَوْفَ يَعْلَمُونَ

Who make [equal] with Allāh another deity. But they are going to know.

Translation: Saheeh International

Tafsir

those mockers who set up besides God another god this entire clause is an adjectival qualification; but it is also said to be the subject and because there is an implicit conditional meaning in the above-mentioned clause the particle fā’ has been inserted into the predicate which is the following soon they will know …

Excerpt from Tafsir al-Jalalayn, commentary on 15:96, via Quran.com. The reader has the full commentary and 9 more tafsirs.