Is it permissible to break the fast in Ramadan due to severe hunger and thirst that affect movement and study, and is it considered a sin to break the fast for a day due to an inability to bear the fasting?

Fasting Ramadan is one of the pillars of Islam, and intentionally breaking the fast without a valid শরعي (Shar'i - religiously lawful) excuse is a major sin. Hunger and thirst are not excuses for breaking the fast unless the fasting person fears death or illness for themselves. If you broke your fast out of fear of harm, there is no sin upon you, but you must make up the missed day. However, if you broke your fast merely due to fatigue and hunger without fearing harm, you have committed a grave sin, and you must repent and make up the missed day. The effect of fasting on physical activity does not permit breaking the fast unless the person is compelled to work and would be harmed by abstaining from it.