Haazinu is a poem that was written for the Jewish people to read in exile to understand why they were punished by HaShem. There is a difficult verse in the beginning of the perashah that talks about the Jewish people's corruption that led to their exile.
The pasuq said:
שחת לו לא בניו מומם דור עקש ופתלתול
שחת means being corrupt or destroying
מומם is their blemish
דור עקש ופתלתול is a stubborn and crooked generation
The question is how to put the whole verse together.
Onkalos translated they destroyed themselves, not HaShem. According to this translation, HaShem is missing from the verse. Meaning לו refers to themselves and לא means not HaShem. Onkalos put בניו with מומם translating that they are children who worship mistaken gods. And Onkalos concluded with a generation that changed their actions and became changed by their actions.
However Rab Saadyah Gaon translated, They were corrupt in front of HaShem, not like his children, they are blemished with stubbornness and crookedness. Similarly Ibn Ezra explained: שחת לו they were corrupt towards Him meaning to HaShem. Whereas Onkalos explained לו as referring to themselves, Rab Saadyah and Ibn Ezra explained that it is referring to HaShem. Then Rab Saadyah and Ibn Ezra put לא with בניו meaning they became לא בניו, not His children. Ibn Ezra then explained that not being His children was מומם, their blemish.
We find yet another explanation from Ramban. He explained, their blemish corrupted them, the non-children who were a stubborn and crooked generation. So like the Ibn Ezra, Ramban put לא with בניו to mean non-children, but he explains לו as referring to themselves like Onkalos.
The Abarbanel offered several more explanations:
1) They corrupted themselves, Hashem did not corrupt them, and their blemish is that they're no longer Hashem's children, they are a stubborn and crooked generation. Here לו is themselves (like Onkalos), לא is with בניו (like Rab Saadyah and Ibn Ezra), and their blemish, also like Ibn Ezra is that they are no longer like HaShem's children.
2) The non-children desecrated Hashem's Name because of their blemish of being stubborn and crooked. Here לו is referring to Hashem's Name. לא is with בניו. And מומם their blemish is described at the end of the verse (stubborn and crooked).
3) Abarbanel also suggested that the verse could be read as a question and answer.
שחת לו? Did they harm HaShem? לא No. בניו מומם the blemish is on the children meaning the Jewish People who are stubborn and crooked.
So שחת לו can either refer to the Jewish People destroying themselves or acting in a corrupt manner towards HaShem, לא is either not connected to another word in the verse or it's connected to the next word which is בניו and means non-children, and מומם (their blemish) is either that they act like non-children of HaShem or that they are stubborn and crooked as specified in the conclusion of the verse. Furthermore בניו could be connected to מומם, meaning the blemish is on the children (the Jewish People), they can not attribute their problems to HaShem. In any case the verse criticizes corrupt behavior, describing it as rebellious and blemishing, and caused by the undesirable character traits of stubbornness and crookedness. This is an important lesson to review especially when we read Ha-azinu on the 9th of Ab and again around the time of Yom Kippur.