Conditionals
You think Emma should book a seat on the train. Otherwise, she has to stand.
If Emma doesn't book a seat on the train, she'll have to stand.
You didn't know how unpopular Jason was when you invited him to your party.
If I'd/I had known how unpopular Jason was, I wouldn't have invited him (to my party).
Warn your friend not to put too many tins into the plastic bag or it'll break.
If you put too many tins into the plastic bag, it'll break.