How can one please a deceased parent and seek forgiveness and pardon from him after having spoken harshly to him?

Allah commanded that parents be addressed with politeness and gentleness, and He forbade scolding them. The Almighty said: (So do not say to them, "uff," and do not repel them but speak to them a noble word. And lower to them the wing of humility out of mercy and say, "My Lord, have mercy upon them as they brought me up [when I was] small.") A noble word is a gentle and kind one, like the saying of a sinful servant to a harsh and stern master. If a son is keen on honoring his parents and strives to seek their pleasure, then any shortcomings on his part are considered grounds for Allah's forgiveness, as He said: (Your Lord is most knowing of what is within your souls. If you should be righteous, then indeed He is ever Forgiving to the oft-returning [to Him].) Allah forgives slips and sudden mistakes for one who is righteous and sincere in his intention to honor his parents.

A person can make up for what he missed in honoring his parents after their death by supplicating and seeking forgiveness for them, giving charity on their behalf, maintaining ties of kinship through them, and honoring their friends.