1. If the film star hadn't died tragically, she wouldn't have become a legend.
2. Dinosaurs wouldn't have become extinct if a sudden catastrophe hadn't happened.
3. I wouldn't be here if my mum and dad hadn't met.
4. If Carlos had concentrated in class, now he could have done his homework.
5. If I hadn't understood the grammar lesson, I wouldn't have been able to / couldn't do this exercise now.