The next three verses (56-58), beginning from: أَن تَقُولَ نَفْسٌ يَا حَسْرَتَىٰ (Pity on me, because I fell short in respect of Allah... 39:56) and concluding on: مِنَ الْمُحْسِنِينَ (...become one of those who are good in deed - 39:58), have elaborated and emphasized the subject of the three verses earlier to it (53-55), that no sinner of any description should despair of the mercy of Allah, for should he repent, Allah will forgive all his past sins. But through the use of the expression: أَن تَقُولَ نَفْسٌ (lest someone should say) at the head of the three verses: 56, 57 and 58, it was reminded that the time of taubah has a deadline - it has to be well before death. If someone were to think of making his taubah after death on the Day of Judgment, or is simply filled with remorse over what awful things he had done to himself, then, that will bring him no benefit.
In this context, it has been mentioned that some disbelievers, on the Day of Judgment, would express different wishes, regretting what they had done. Someone will be filled with remorse as to why did he have to fall short in obeying the commands of Allah Ta’ ala. Then, there will be that odd person even at a place like that who would love to wriggle out of the impasse blaming his destiny for his misconduct by saying - 'Had Allah Ta’ ala given me the necessary guidance, I too would have been one of those who feared Him, but since He gave me no guidance, there was nothing I could do about it.' Still someone else would wish: 'Would that I were to be sent back into the world to become an observing believer, obeying the commandments of Allah at its best.' But, that will be a time when no wish and no remorse of any kind will work.
These three different wishes could belong to different people, and it is also possible that these three wishes could have come, one after the other, from a single group of disbelievers, because, with the last saying which mentions the wish to be sent back to this world, it has been said in the verse that it will come to pass after they had seen the punishment. This obviously suggests that the first two sayings belong to a time before they saw the punishment, that is, it will be on the very first day of Qiyamah that they will remember the shortcomings of their deeds and say: يَا حَسْرَتَىٰ عَلَىٰ مَا فَرَّطتُ فِي جَنبِ اللَّـهِ (Pity on me, because I fell short in respect of Allah ...39:56). Later, as an excuse, they will say that they were helpless, and had Allah guided them, they too would have become obedient and fearing, but when He Himself gave them no guidance, they were hardly at fault. After that, when they see the punishment, they would wish to be sent back into the world. In these three verses, Allah Ta’ ala has made it very clear that the forgiveness and mercy of Allah is very extensive. But, it can be won only when one repents before death. Therefore, Allah was alerting them right there against the possibility that they start to show remorse after death, and indulge in these redundant wishes in the Hereafter.