وَيْلٌ يَوْمَئِذٍ لِّلْمُكَذِّبِينَ
And its position is meaningful: it comes after the proof from history. The section had ended "thus do We deal with the guilty"; the refrain ties this to a day.
- 18: kadhālika nafʿalu bi'l-mujrimīn — imperfect, an ongoing practice, without time.
- 19: waylun yawmaʾidhin li'l-mukadhdhibīn — tied to a day.
That is, the section ends with a timeless rule, and the refrain nails that rule to a particular day. This is what the refrain does every time, and it is why yawmaʾidhin's having been fronted mattered.