He can't move his arm. Perhaps it's broken. (might)
He can't move his arm. It might be broken.
It tastes disgusting. I'm sure it isn't fresh. (can't)
It tastes disgusting. It can`t be fresh.
He's got a cough and a sore throat. Perhaps he has a cold. (could)
He's got a cough and a sore throat.He could have a cold.