لِتَسْتَوُوا عَلَى ظُهُورِهِIn order that you may mount on their backs,meaning, sit comfortably and securely,عَلَى ظُهُورِهِ (on their backs) means, on the backs of these kinds of animals.ثُمَّ تَذْكُرُوا نِعْمَةَ رَبِّكُمْand then may remember the favor of your Lord,means, whereby these animals are subjugated to you.إِذَا اسْتَوَيْتُمْ عَلَيْهِ وَتَقُولُواسُبْحانَ الَّذِي سَخَّرَ لَنَا هَذَا وَمَا كُنَّا لَهُ مُقْرِنِينَwhen you mount thereon, and say:"Glory to Him Who has subjected this to us, and we could have never had it."means, if it were not for the fact that Allah has subjugated these things to us, we could never have done this by our own strength.'Ibn Abbas, Qatadah, As-Suddi and Ibn Zayd said:"We could not have done this ourselves."وَإِنَّا إِلَى رَبِّنَا لَمُنقَلِبُونَ