Adam Clarke on Job 29:24

I laughed on them, they believed it not - Similar to that expression in the Gospel, Luk 24:41 : And while they believed not for joy, and wondered, he said - . Our version is sufficiently perspicuous, and gives the true sense of the original, only it should be read in the indicative and not in the subjunctive mood: I laughed on them - they believed it not. We have a similar phrase: The news was too good to be true.

The light of my countenance - This evidence of my benevolence and regard. A smile is, metaphorically, the light of the countenance.

They cast not down - They gave me no occasion to change my sentiments or feelings towards them. I could still smile upon them, and they were then worthy of my approbation. Their change he refers to in the beginning of the next chapter.