modal verbs

Have to	should	must
Have to" is used to express certainty, necessity, and obligation.	"Should" is most commonly used to make recommendations or give advice.	Must" is most commonly used to express certainty.
Behaves quite irregularly in the past and the future.	It can also be used to express obligation as well as expectation	It can also be used to express necessity or strong recommendation, although native speakers prefer the more flexible form "have to."