1) IF PRESENT  ---- WILL + VB 

• if the weather is nice, I will go to the beach 
• if I have a lot of money, I will buy a castle

2) IF PAST  ---- WOULD + VB 

• if the weather were nice, I would go to the beach
• if I had a lot of money, I would buy a castle

3) IF PAST PERFECT  ---- WOULD HAVE + PP 

• if the weather had been nice, I would have gone to the beach
• if I had had a lot of money, I would have bought a castle