1 you should take exercise regularly because that's the way to get fit.
if you take exercise regularly, you'll get fit.
2 i can't look the word up because i haven't got a dictionary.
if i had got a dictionary, I could look the word up.
3 i'm so busy, so i don't write to my friends regularly.
if i wasn't so busy, I would write to my friends regularly.
4 Sandra may arrive a bit early. she can help you get things ready.
if Sandra arrive a bit early, she can help you get things ready.
5 it is raining, so we won't finish the game.
if it wasn't rain, we would finish the game.