1. You are not to tell anyone about your plans

или

You must not tell anyone about our plans.

2. There must be some kind of a mistake with the phone bill - it's astonomical.

3. Your brother must have been speeding if he got fined. (За това изречение има по-голяма вероятност да бъркам, отколкото за другите.)

4. We had to collect iron every month when I was at school.