Do major sins always invalidate the reward of good deeds, or only sometimes, and when do they invalidate them? And do minor sins also invalidate deeds?

Those who commit major sins are subject to Allah's will. If He wills, He will punish them, and this is the meaning of their major sins nullifying their good deeds. Or He may forgive them out of His mercy. Complete nullification of good deeds only occurs through polytheism (shirk).

Al-Hafiz Ibn Hajar said: "Nullification of deeds means deprivation of reward, because reward is only for what is done with sincerity."

Qadi Abu Bakr Ibn Al-Arabi said: "Nullification is of two types: nullification of one thing by another, causing its complete removal, like the nullification of faith by disbelief, and disbelief by faith. And nullification through balancing. So, whoever's good deeds outweigh their bad deeds is saved. And whoever's bad deeds outweigh their good deeds is subject to Allah's will; He may either forgive them or punish them. This is a relative nullification, not an absolute one."

As for minor sins, they are expiated by avoiding major sins, as Allah Almighty says: "If you avoid the major sins which you are forbidden, We will remove from you your lesser sins and admit you to a noble entrance [into Paradise]." [An-Nisa:31]. However, persistence in committing them may transform them into major sins.