Translation:

48. If then they turn away We have not sent thee as a guard over them. Thy duty is but to convey (the Message). And truly when We give man a taste of Mercy from Ourselves He doth exult thereat but when some ill happens to him on account of the deeds which His hands have sent forth truly then is man ungrateful!

Notes (Tafseer)

4593. The warning is now given, that men may repent and do good, and pray for Allah's Mercy and Grace. If the warning is not heeded or is rejected, the prophet of Allah is not responsible for bringing about the Penalty or for forcing people to come to the right Path. He is not a guard set over them to free them from the need of exercising their limited free-will.

4594. Cf. xxx. 36. It is a sad reflection that men, when they receive some gift out of Allah's Mercy, exult in their good fortune and attribute it to some merit in themselves, instead of to the Grace and Mercy of Allah, thus missing the real lesson of Life. On the other hand, when they are in trouble, due to their own errors and shortcomings, they fall into despair and blame Allah, instead of blaming themselves. This is rank ingratitude. So they miss the true lesson of Life in that case also.