=>

1. If they understood the problem, they would find a solution.

2. You will be late for the train if you don't go right now.

3. They would understand you if you didn't speak English unnaturally./They would understand you if you spoke English naturally

1. If they don't understand the problem , they won't find a solution

2. If you don’t go right now, you’ll be late for the train.

3.  If you spoke English naturally, they’d understand you.

4.If we had known your phone number, we could have contacted you.

5. If Simon lived near his mother, he could visit her often.

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

@  If they understood the problem, they would find a solution.

2. Go right now or you’ll be late for the train.

@  You will be late for the train if you don't go right now

3. Because you speak English unnaturally, they do not understand you.
They would understand you if you didn't speak English unnaturally.
They would understand you if you spoke English naturally.