What is the ruling on sometimes delaying and combining prayers due to studying or fatigue, and is that considered a major sin? And what is the way to achieve tranquility and confidence in entering Paradise, despite the fear of falling short?

Delaying prayer beyond its preferred time is one of the major sins, as Allah Almighty says: "So woe to those who pray, [but] who are heedless of their prayer" and "Then succeeded them a posterity who neglected prayers and pursued desires; so they are going to meet evil." Studying is not an excuse for delaying prayer, and one must repent from this sin and avoid it in the future. If university circumstances do not permit performing prayer on time, then one must leave the university. Excessive worry about shortcomings in worship serves as a motivator to increase good deeds, while combining fear and hope for Allah's mercy is obligatory, as in His saying: "fears the Hereafter and hopes for the mercy of his Lord." Lack of reassurance may be due to delaying prayer, as it is a turning away from the remembrance of Allah, and "whoever turns away from My remembrance - indeed, he will have a difficult life."