Word by Word Analysis

وَأَقْسَمُوا۟ بِٱللَّهِ جَهْدَ أَيْمَـٰنِهِمْ لَئِن جَآءَهُمْ نَذِيرٌ لَّيَكُونُنَّ أَهْدَىٰ مِنْ إِحْدَى ٱلْأُمَمِ ۖ فَلَمَّا جَآءَهُمْ نَذِيرٌ مَّا زَادَهُمْ إِلَّا نُفُورًا 42

And they swore by Allāh their strongest oaths that if a warner came to them, they would be more guided than [any] one of the [previous] nations. But when a warner came to them, it did not increase them except in aversion