1. She doesn’t live in London because she doesn’t know anyone there

 If she knew anyone in London, she would live there.

2. I don’t have a spare ticket. I can’t take you to the concert

 If I I had a spare ticket, I could take you to the concert

3. The schoolchildren sowed some seeds, but they forgot to water them so they didn’t grow.

 If the children didn't forget to water seeds, they would grow.

4. They don’t understand the problem. They won’t find a solution

 If they understood the problem, they would find a solution