past possibilities

Some people believe that aliens MAY HAVE/MUST HAVE helped the Egyptians build the pyramids thousands years ago.

People saw the river turn red yesterday. Wastes from a nearby factory MAY HAVE/MUST HAVE caused it. The police are still investigating the phenomenon.

The creature I saw yesterday looked like a dog, but it had spikes on its bag. It COULDN'T HAVE/MUST HAVE been a dog. Dogs don't have spikes on their backs

There 's a possibility that prehistoric people MIGHT HAVE/MUST HAVE built the pyramid structure of Gunung Padang 20,000 years ago.

I heard my grandma's voice in my room last night. I MUST HAVE/MUSTN'T HAVE been dreaming because my grandma passed away years ago.

A ghost COULDN'T HAVE/MAY HAVE stolen your watch. You MUST HAVE/COULDN'T HAVE forgotten where you had put it.

I think I saw Bigfoot crossing the street in front of me last night, but I'm not sure. I MUST HAVE/MIGHT HAVE seen a big bear.

Scientists are still debating about how the stones were moved to build the pyramids in Egypt. Some believe that the pyramid workers MUST HAVE/MAY HAVE pushed and rolled the stones

There's no evidence or research confirming the age of the artifact yet. It MUST HAVE/MIGHT HAVE been from the medieval era.

I MAY HAVE/MUST HAVE gone completely white because my friends knew something was wrong with me right away.