The following modal verbs can be used in conditional sentences in the Subjunctive mood:
Could
If the action refers to the past could is followed by the Perfect Infinitive. Could - expresses possibility.
But for the bad weather we could go to the country.
Несмотря на плохую погоду, мы могли бы поехать загород.If I had had money about me yesterday, I could have bought this book.
Если бы у меня вчера были с собой деньги, я мог бы купить эту книгу.Might
Might expresses possibility (could is more common here) or somebody's supposition.
If he asked me for help, I might help him.
Если бы он попросил меня о помощи, я бы мог помочь ему.If Nick had proposed to me then, I might have married him.
Если бы Ник тогда предложил мне, я возможно вышла бы за него.If the action refers to the past might is followed by the Perfect Infinitive.
To have
To have expresses necessity.
If I weren't ill, I should have to go on this business trip.
Если бы я сейчас не был болен, мне пришлось бы поехать в эту командировку.If had not stayed in Switzerland, I should have had to take part in this project.
Если бы я не задержался в Швейцарии, мне пришлось бы принять участие в этом проекте.Would
Would expresses wish or resolution to perform an action.
If you would join my group, I would give you a very interesting job.
Если бы ты пожелал перейти в мою группу, я непременно поручил бы тебе очень интересную работу.