Modal verbs are easy to spot because there are so few of them. In addition to must, shall, will, should, would, can, could, may and might, we can add ought to and have to. If we are told that we 'must ...

A simple sentence is built from the minimum of a subject and a main verb. It can be very short in length (but doesn't have to be): 'The angry dog barks.' It puts across one simple idea: 'Rebecca sang.