Ответ:
1. Jane had to stay at home because it was very cold.
Did Jane have to stay at home because it was very cold?
Jane didn't have to stay at home because it was (not) very cold.
2. Jane was allowed to go skiing to the park.
Was Jane allowed to go skiing to the park?
Jane wasn't allowed to go skiing to the park.
3. The generator must be repaired as soon as possible.
Must the generator be repaired as soon as possible?
The generator doesn't have to be repaired as soon as possible
4. Light waves may be reflected.
May light waves be reflected?
Light waves may not be reflected.
5. A great number of plastics should find their application in the electrical industry.
Should a great number of plastics find their application in the electrical industry?
A great number of plastics shouldn't find their application in the electrical industry.