Look at these sentences and explain the difference between them. Which mean the same? It might have been a ghost. It must have been a ghost. It could have been a ghost. It can't have been a ghost.
The difference is from the semi-modal verbs.The first three are positive,but the second expresses certainy,while the other two don’t.The fourth is negative,but it’s not certain. Also,the first and the third have the same meaning.
