John would surely not have gone to school yesterday if he had injured his leg, but we do not know if he did. Its fine, a bit clumsy but understandable.

I would say: but we did not know WHETHER he did OR NOT. That just sounds more natural

He wouldn't have gone to school yesterday if he had injured his leg, but we don't know that he has. Has could only refer to injur. He wouldn't have gone to school yesterday if he had injured his leg, but I don't know if he did. Did leads me to think of "gol" but it could also refer to injur. to remove any ambiguity: He wouldn't have gone to school yesterday if he had injured his leg but I don't know if he (has) injured it. He wouldn't have gone to school yesterday if he had injured his leg but I don't know if he went.

Ramond, You are right, but I think" injured his leg "is also ambiguous a bit. What I understand about it is that the subject harmed his self. I think it's better if "injured leg" . The word injured serves as adjective that discribes the situation of his leg.

Hi. Is this sentence OK? Do you see any contradiction in it? (?) required I think you are correct to make contradiction singular but something makes me feel it should be plural... any teachers about?