1.  Paul is only 15 but he wants to leave school. If he (leave) now he (be) sorry afterwards.

Paul is only 15 but he wants to leave school. If he LEAVES now he WILL BE sorry afterwards.

2.  Unless my son (take) his library book back tomorrow he (have) to pay a fine.

Unless my son TAKES his library book back tomorrow he WILL HAVE to pay a fine. 

3.  Your wife can (use) my smartphone if hers (not work).

Your wife can USE my smartphone if hers IS NOT (ISN'T) WORKING. 

4.  If someone (ring) while I am at the supermarket, please say that I (be) back by 9 pm.

If someone RINGS / SHOULD RING while I am at the supermarket, please say that I WILL BE back by 9 pm.

5.  If your sister Amelia (care) to see some of my daughter's paintings I (send) three to your home.

If your sister Amelia WOULD CARE to see some of my daughter's paintings I WILL SEND three to your home.