You have to give the book back. 
=> the book has to be gave back 

We must wash this kind of cloth in warm water only. 
=>  this kind of cloth must be washed  in warm water only. 

You should do your homework. 
=> your homework should be done

Someone might find a solution to the problem soon. 
=> a solution to the problem might be found soon

Someone should tell him never to do it again. 
=> he should be told never to do it again. 

we can keep our fields in good condition. 
=> our fields can be kept  in good condition.