All conditional sentences can be subdivided into those where the action refers to the present or future and those where it refers to the past.
Model 1 – the action refers to the present or future.
The past subjunctive of the verb to be or the Past Indefinite of other verbs is used in the subordinate clause and the analytical Subjunctive consisting of the mood auxiliary should for the first person and would for the second and third persons plus the bare Infinitive of the notional verb is used in the principal clause.
If he were not ill now, they would send him to this business trip.
Если бы он сейчас не болел, они бы отправли его в командировку.
If I had a diploma, I should get this job.
Если бы у меня был диплом, я бы получил эту работу.
Note: There are two special forms, which can be used, in the subordinate clause to show that the action is unlikely to take place. These are the Past Subjunctive of the verb to be plus to-Infinitive and should (for all persons) plus bare Infinitive. These forms can only be used when the action refers to the future.
They can be translated into Russian as: случилось так…, если бы случилось так…, вдруг…
If I were to meet him somewhere, I would tell him what I think about him.
Если вдруг я встретил бы его где-нибудь, я бы сказал все, что о нем думаю.
If two burglars should rob one and the same house on the same night independently, their crime would not be regarded as conspiracy.
Если бы вдруг два грабителя независимо друг от друга ограбили бы один и тот же дом в одну ночь, их преступления не рассматривались бы как сговор.
Model 2 - the action refers to the past.
The Past Perfect is used in the subordinate clause and the analytical Subjunctive consisting of the mood auxiliaries should and would accordingly plus the Perfect Infinitive is used in the principal clause.
If you had helped me yesterday, we should have done this work.
Если бы ты помог мне вчера, мы бы сделали эту работу.
There are the following rules here: