Grammar — Modals

"In Britain, you must drive on the left."

ما الفرق بين must وhave to في التعبير عن الإلزام؟

"He has a backache. He shouldn't carry heavy things."

ما الفرق بين shouldn't وmustn't في التعبير عن النهي؟

"If he has a credit card, he doesn't have to pay in cash."

ما معنى don't have to؟ وكيف يختلف عن mustn't؟

"Tom doesn't study enough. He should study harder."

ما وظيفة should في هذه الجملة؟ وكيف يختلف عن must؟

"You must wear sports shoes. You don't have to be silent."

كيف نُميّز بين الإلزام وغياب الإلزام في الجمل السابقة؟