`1`. If I had saved my pocket money, I could have bought Bob a present.
`->` If 3: If + S1 + had + Vpp, S2 + could/ would have + Vpp
`2`. If I hadn't lost the invitation, I wouldn't have missed his party.
`->` If 3: If + S1 + had + Vpp, S2 + could/would have + Vpp
`3`. If I had his number, I could call him.
`->` If 2: If + S1 + V2/ed, S2 + would + Vinf
`4`. If I see him tomowrrow, I'll give him a birthday card.
`->` If 1: If + S1 + Vs/es, S2 + will + Vinf
