Irrealis conditions

In these constructions, the condition clause expresses a condition that is known to be false, or presented as unlikely. The result clause contains a conditional verb form consisting of would (or could, should, might) plus an infinitival main verb.

The second conditional is used to refer to a current state or event that is known to be false or improbable. The past subjunctive (or in colloquial English, simply the past tense) must be used:

If she were [colloq. was] at work today, she would know how to deal with this client.
If I were [colloq. was] king, I could have you thrown in the dungeon.

The same structure can be used to refer to a future state or event:

If I won the lottery, I would buy a car.
If he said that to me, I would run away.

In many cases, when referring to future events, the difference between a realis and irrealis conditional is very slight:

(realis) If you leave now, you can still catch your train.
(irrealis) If you left now, you could still catch your train.

The third conditional is used to refer to contrary-to-fact past events. The pluperfect (or past perfect) is used in the condition clause.

If you had called me, I would have come.
If you had done your job properly, we wouldn't be in this mess now.

Note that would-conditional forms are not usually used in the condition clause in English: *If you would leave now, you would be on time. There are exceptions, however: If you would listen to me once in a while, you might learn something. Some varieties regularly use would have in the protasis for past reference: If you would've told me, we could've done something about it.

Should can appear in the condition clause to refer to a future event presented as possible, but unlikely, undesirable, or otherwise "remote": If I should die before I wake, …, If you should ever find yourself in such a situation, …