If I won a million dollars, I would buy an island

If we are allowed to take part in the competition, we'll take the chance

If you have flu, the doctor doesn't allow you to go to school (Если у тебя грипп, врач не разрешает тебе ходить в школу) ЛИБО
If you have flu, the doctor will not allow you to go to school (Если у тебя будет грипп, врач не разрешиит тебе ходить в школу) Подходят оба варианта

If he wins the swimming competition, he will be invited to join the national team

What would you ask the Olympic champion if you could meet him in person? (слово в скобке не видно, предположительно CAN)

If I were you, I wouldn't have fallen ill. Your cold is not as harmless as you think.