Did you know that in Russian there are only two types of conditionals, the real one and the unreal one. The unreal one can refer to a present or to a past action, you can only understand it from the general context.
Если ты пойдёшь в театр, я пойду с тобой. If you go to the theatre, I will go with you. Real conditional.
Если ты бы пошёл в театр, я бы пошёл с тобой. If you went to the theatre, I would go with you. Unreal conditional, action in the present.
Если бы ты тогда пошёл в театр, я бы пошёл с тобой. If you had gone to the theatre, I would have gone with you. Unreal consitional, action in the past.