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.
You haven't got a pen, so you can't write down the address.
If I had a pen, I could write down the address.
You should have started your project earlier. You're so far behind last week.
If I'd/I had started my project earlier, I wouldn't have been so far behind last week.
Your friend might need some help. If so, tell her to give you a ring.
If you need some/any help, give me a ring.
The automatic result of the door opening is the fan coming on.
If the door opens, the fan comes on.
I haven't got a key, so I can't get in.
If I had a key, I could get in.
You talk about football and I'm going to be very bored.
If you talk about football, I'm going to be very bored.
The baby didn't cry, so we had a good sleep.
If the baby had cried, we wouldn't have had a good sleep.
You may want a chat, so just give me a ring.
If you want a chat, just give me a ring.
Nigel hasn't got satellite TV, so he can't watch the game.
If Nigel had satellite TV, he could watch/would be able to watch the game.
You go away and I'll miss you.
If you go away, I'll miss you.
I'm afraid of spider, so I can't pick it up.
If I wasn't afraid of spiders, I would pick it up.
Throw a stone into water and it sinks.
If you throw a stone into water, it sinks, OR it'll/it will sink.
We got lost because we didn't look at the map last year.
We wouldn have got lost if we had looked at the map.
