Counterfactual

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 a main verb in the base form (infinitive without to).
The contrary-to-fact present conditional, often referred to as the "second conditional" or "conditional 2", 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] the 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 contrary-to-fact past conditional (sometime referred to as the "third" conditional, conditional 3) 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.

Some varieties of English regularly use would (often shortened to (I)'d) and would have (often shortened to (I)'d have) in counterfactual condition clauses, but this is often considered non-standard: If you'd leave now, you'd be on time. (conditional 2.) / If you (would)'ve told me, we could've done something about it. (conditional 3.) Such use of would is widespread especially in spoken US English in all sectors of society, but these forms are not usually used in more formal writing. Nevertheless, some reliable sources simply label this usage as acceptable US English and no longer label it as colloquial.^[3][4]
There are exceptions, however, where would is used in British English too in seemingly counterfactual conditions, but these can usually be interpreted as a modal use of would: If you would listen to me once in a while, you might learn something. (conditional 2.) ^[1][2] In cases in which the action in the if clause takes place after that in the main clause, use of would in counterfactual conditions is however considered standard and correct usage in even formal UK and US usage: If it would make Bill happy, I'd [I would] give him the money.^[1]
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, …