Tenses in conditional sentences

General rule:

If + present + present

If the temperature reaches 100 ^oC, water boils.

If it rains, the children play inside.

Future possibility:

If + present + future

If I get a raise, I will take you out to dinner.

If it rains tomorrow, the children will play inside.

Actual or possible past situation: Various tenses are possible.

If + past + present

If I was too blunt, I apologize.

If + past + past

If Al made a snack, he always cleaned the kitchen.

If + past + conditional

If Sarah was working in the garden, she couldn't have heard the phone.

If + present perfect + future

If any trees have blown down, we'll need to clear the road.

Unlikely, imaginary or impossible situation (present or future):

If + simple past + would…

If Ed skied this winter, he would buy new boots.
[but he probably won't ski]

If Pia had the money, she would take a cruise.
[but she doesn't have it; she can only dream]

If you got up earlier, you'd be on time.
[but you probably won't get up earlier]

If wishes were horses, beggars would ride.
[but that's impossible]

Note: For the verb be, Canadians prefer were with all subjects in this type of clause:

If I were invited, I would go.

If Gayle were here, Lena would be so pleased.

Past situation that did not happen:

If + had… + would have…

If Sue had played, we would have won. [but she didn't]

If Karl had insisted, I would have gone with him. [but he didn't]

Note: Do not use would have in the if clause:

Weak: If you would have asked me, I would have helped.

Correct: If you had asked me, I would have helped.