As it is already explained in the chapter of verb that modal operators are those auxiliaries which have only one or two tensed forms. They are also called modal auxiliaries or modals in short, they are;
can, could, may, might, ought to, shall, will, would, must, should.
If any sentence has modal operator as mentioned above you have to convert it into passive voice by following the simple instructions below.
Subject converts into object.
Use of auxiliary
As for auxiliary the same modal operators are used followed by be.
I couldn’t understand you.
We can make mobile software.
You may operate this machine.
He might tell his secret.
You ought to practice law.
I shall learn English.
He will break rules.
I would read newspaper every morning.
You must follow these instructions.
You should read more books.