1.can 可能,可以,、can I sit here? yes ,you can.
can't 、不能,不会 ,表示能力.I can't speak English.
can be 、可能会,表示理论上的可能性 The weather in north China can be a little colder than that in the south.
can't be不可能是表示否定推断.it can't be his sweater,because his is black.
2.must 、必需,一定 you must study hard.
mustn’t、一定不要,you mustn't play with fire.
must be 一定是表示推断.he must be in the house ,for the lights are on.
3.have to 不得不,表示被动的必需.You have to get up early if you want to catch the early bus.
4.need to需要做..we need to wear warm clothes to protect us from the cold in winter.