1. I did't see the signal so Idid't stop

=> If I had seen the signal, I would have stopped.

2.she did't know you were in hospital so she didn't visit you

=> If she had known you were in hospital, she would have visited you.

3. I did't know your number so I didn't ring

=> If I had known your number, I would have rung.

4.We only came by bus because there were no taxis

=> If there had been some taxis, we wouldn't have come by bus.

5.She didn't speak to him possible because she was so shy

=> If she hadn't been so shy, she would have spoken to him.