[1] Allah’s punishment is imminent and certain, as the Hour is approaching.
[2] Shirk (associating partners with Allah) is the most egregious of all sins, as He warned against it right after mentioning the imminence of the Hour. Allah stresses against Shirk or calls for pure Tawhīd (monotheism) in the two subsequent verses as well.
[3] As soon as he becomes strong and independent. The ones who are meant here are, especially, the disbelievers who deny resurrection, argue against the Truth, and are ungrateful to Allah’s favors.
[5] Their bringing back is mentioned before their setting off to pasture because that is when their udders and waists are full and thus more pleasing to the beholder.