Adapted from Betty Azar's Fundamentals of English Grammar Book
TRUE IN THE PRESENT
IF clause = simple present, RESULT clause = simple present/future
If I visit Italy this year, I will go to the Trevi Fountain.
If the weather is nice today, we can go to the park.
If the snow falls by morning, we can go skiing.
If I have enough time, I will write a travel book.
NOTE: Superstitions often use this tense.
If you knock on wood, your good luck will continue.
UNTRUE IN THE PRESENT/FUTURE
IF clause = simple past, RESULT clause = would / could + simple form
If I visited Italy today, I would go to the Trevi Fountain.
If the weather were nice today, we could go to the park.
If the snow fell today, we could go skiing.
If I had enough time, I would write a travel book.
NOTE: This tense is often used for this example.
If I were you, I would accept the invitation.
UNTRUE IN THE PAST
IF clause = past perfect, RESULT clause = would have + participle
If I had visited Italy this year, I would have gone to the Trevi Fountain.
If the weather had been nice yesterday, we could have gone to the park.
If the snow had fallen by morning, we could have gone skiing.
If I had had enough time, I would have written a travel book.