Is zakat due on grains and money from one whose father left large debts with high interest, and who can barely cover his study expenses and bank loan, and lives in his grandfather's house?

If the loan was usurious, then you must seek forgiveness for your father, and you are not required to repay the usurious interest; rather, dispose of it if possible. However, if it was not usurious, then the loan must be repaid from your father's estate. If he has no estate, then the heirs are not obligated to repay it. If you voluntarily donate to repay it, you are not permitted to deduct the amount from zakat funds because it is not a debt binding upon you. There is a difference of opinion regarding the permissibility of giving zakat to your deceased father's creditor. However, if the debt has become binding upon you and you have committed to repaying it, there is a difference of opinion among scholars regarding deducting this debt from zakat funds.