You should have started your project earlier. You're so far behind now.
If I had started my project earlier, I wouldn't be so far behind (now).
I haven't got a key, so I can't get in.
If I had a key, I could get in.
We should have bought that vase five years ago because it would be quite valuable now.
If we had bought that vase five years ago, it would be quite valuable now.
I'm not rich or I'd have given up working long ago.
If I was/were rich, I would have given up working long ago.
He's very ill so he isn't at work.
If he wasn't very ill, he would be at work.
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.
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.
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 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 (to my party).
You go away and I'll miss you.
If you go away, I'll miss you.
You talk about football and I'm going to be very bored.
If you talk about football, I'm going to be very bored.
You may want to chat, so just give me a ring.
If you want to chat, just give me a ring.
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.
We're lost because we didn't look at the map.
We wouldn't be lost if we had looked at the map.
I'm afraid of spider or I'd have picked it up.
If I wasn't afraid of spiders, I would have picked it up.
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.
Throw a stone into water and it sinks.
If you throw a stone into water, it sinks, OR it'll/it will sink.