For those who responded, the 'lam' is related to 'He strikes', meaning that Allah strikes examples for the believers who responded, and for the disbelievers who did not respond. That is: they are like the two groups. And 'the best' is an attribute of the source of 'responded', meaning: they responded with the best response. And His saying 'If only they had' is an introduction to mentioning what is prepared for those who do not respond. It is said: the speech has ended at the saying 'Thus Allah strikes examples' and what follows is a new statement. 'The best' is the subject, and its predicate is 'for those who responded', and the meaning is: for them is the best reward, which is Paradise. And 'those who did not respond' is the subject with its predicate. 'If' with what is in its scope and 'the evil of the reckoning' is discussed therein. And from Al-Nakha'i: that a man is held accountable for all his sins, and nothing is forgiven from them.