فَبَشِّرْهُم بِعَذَابٍ أَلِيمٍ
Sentence structure
In فَبَشِّرْهُم, بَشِّرْ is a Form II imperative with necessarily understood أَنْتَ as subject and attached هُم as object. بِعَذَابٍ is linked to it and أَلِيمٍ is a genitive adjective.
Ayah summary
Give them news of a painful punishment as the consequence of their deeds.
فَبَشِّرْهُم — so give them tidings
Grammar: فَ is causal/resumptive, بَشِّرْ a Form II second-person masculine-singular imperative with understood أَنْتَ as subject, and attached هُم its object.
بِعَذَابٍ — of a punishment
Grammar: بِ is a preposition and عَذَابٍ an indefinite genitive noun; the phrase is linked to بَشِّرْ.
أَلِيمٍ — painful
Grammar: An indefinite masculine-singular genitive adjective describing عَذَابٍ.