Conditionals
I didn't hear you because I had my headphones on. If ...
If I hadn't had my headphones on, I would have heard you.
She is a shy girl, so she doesn't go out very often. If ...
If she wasn't such a shy girl, she would go out more often.
The weather looks good, so they might go sailing at the weekend. If ...
If the weather is good, they will / might go sailing at the weekend.