>1. I'm sure my parents are working now.
>My parents must {be} working now. - czynnosc dotyczy obecnej sytuacji, stad 'must be + v-ing'
>
>2. Nie musiałam iść wczoraj do szkoly.
>I didn't have to go to school yesterday. OK
>
>3. I'm sure he told her.
>He must have told her. OK
>
>4. John niepotrzebnie wstał o 6 rano.
>John needn't have {got} up at 6 o'clock.
>
>5. I'm sure it was Mark who borrowed the book.
>It must have been Mark who borrowed the book. OK
>
>6. I expect you are exhausted after your long trip.
>You must be exhausted after your long trip. OK