How do we respond to those who deny the torment of the grave, arguing that nothing changes in the grave?

The answer to one who denies the torment of the grave is that the torment of the grave is affirmed by Islamic law. Allah Almighty said regarding the people of Pharaoh: "The Fire, they are exposed to it morning and evening. And the Day the Hour is established [it will be said], 'Admit the people of Pharaoh to the severest punishment.'" This is also affirmed in the Sunnah of the Prophet. Therefore, it is not permissible to contradict these texts with mere conjecture.

The torment of the grave is primarily upon the soul, and it is not physically perceptible to the body. It is among the matters of the unseen that cannot be measured by the conditions of this world. The deceased is the one who perceives it, not others, just as a sleeping person sees things in their dream that those around them do not feel. The Muslim's duty in such matters is submission and faith.