We use must, could, might and can't + infinitive to talk about possibility in the present.

We use must + infinitive when we are sure that something is correct in the present. 
Ex. You haven't eaten all day. You must be hungry!
Ex. John hasn't arrived yet. He must be stuck in traffic. 

We use could, may or might + infinitive when something is possible in the present. 
Ex. The man in the photo could be American, he has a big hat. 
The book might be on the table. 

We can also use could, may or ming to talk about future possibility
Ex. He might not text me tonight 
Ex. We could catch the bus to college tomorrow

We use can't + infinitive when something is impossible
Ex. You can't be cold – it's 28 degrees out there!
Ex. He can't be tired. He hasn't done anything all day!