Sue didn't get me a birthday present, which is why I didn't get her one.
If Sue had got me a birthday present, I would have got her one.
We didn't pay them on time because they made a mistake with our order.
If they hadn't made a mistake with our order, we would have paid them on time.
You didn't know how unpopular Jason was when you invited him to your party.
If I had known how unpopular Jason was, I wouldn't have invited him.
Your colleague went to the party and didn't finish his project on time. (You're frustrated with his action)
You shouldn't have gone to the party!/You should have finished your project on time!
Shortly after you started your own business, a multinational offered to acquire it but you refused. Now you have some regrets. (Use "If only..")
If only I had accepted the offer/hadn't refused
There was a chance to get a better position but you missed it (Use "if only" and "should have")
I shouldn't have missed a chance to get a better position!/If only I hadn't missed...
Your friend didn't tell you that he was going to be late. (Express criticism)
You should have told me that you....
They didn't reduce prices and thus not many people bought your product.
...
Your friend didn't have good references so he wasn't employed.
...
