Does Allah answer the prayer of someone who supplicates for something that is unknowingly harmful to them, or does He avert the harm and ordain good for them? And does the matter differ with the persistence of the supplicant? And is it necessary to say, "If it is good..." in every supplication?

It is best to adhere to the prophetic supplications and those that combine the good of this world and the Hereafter. If a person supplicates for something that contains evil for him, then Allah Almighty does not answer that supplication. Allah Almighty says: "And if Allah were to hasten for the people the evil [they invoke] as He hastens the good, their term would have been concluded." Ibn Kathir said that Allah, due to His forbearance and kindness to His servants, does not answer them if they invoke evil upon themselves, their wealth, or their children in a state of distress and anger, because He knows that they do not intend evil. Allah Almighty also says: "And man invokes evil as he invokes good, and man is ever hasty." Thus, Allah does not answer everything a servant supplicates for due to his hastiness and his assumption that contradicts reality; and if a person supplicates for what he considers good, but it is evil for him, then Allah does not hasten the answer to him out of mercy for him, but He does not deprive him of the reward of his supplication.