Напишите предложения в past indefinite и future indefinite 1) you must do it now 2) you must learn this rule 3) you can speak to him about it 4) she can help you 5) we must go home 6) who can teach him business?

Так как у модального глагола Must нет формы будущего и времени, то в таких ситуациях вместо него используется модальный глагол Have to:

Из-за того, что глагол Can не имеет формы будущего времени, для описания действия в будущем он заменяется на “be able to”:

1) you had to do it. You will have to do it. 
2 ) you had to learn this rule. You will have to learn this rule next week 
3 ) you could speak to him about it.  You will be able to speak to him about it  
4 )she could  help you. She will be able to help you. 
5 ) we had to go home.  We will have to go home     
6) Who could teach him business? Who will be able to teach him business?