What is your advice to an Imam who recites long surahs during audible prayers but shortens them, while the congregants ask him to recite surahs from the thirtieth juz’ on the grounds that they do not know the long ones?

There is no harm for the Imam to recite from the long surahs as long as he lightens the prayer, and he is not obliged to obey the worshippers by restricting himself to the thirtieth juz. We advise him to recite from the short surahs to combine the lightness required by sharia with reciting the entire surah, and to resolve the disagreement. The matter is related to the conditions of the worshippers, and the majority of jurists have held that it is sunnah for the worshipper to recite something from the Qur'an after Al-Fatiha, and it is preferable to recite a complete surah.