I got in trouble and people said "he must have done something" Which one is correct? " must have done something" or "must have something done " do they mean the samethng?
They are correct but completely different. The first one means high probability of a past event seen from the Present. The second one refers to the necessity of having something done by someone. The second construction is a specific example of the Passive Voice. For example, I can't start my car. It must have broken down. I must have it fixed.