Flip evidence for Speculations in the past

A friend promised to return a book he borrowed. He hasn't. He's normally very reliable

• He could 

  have lose it.

• He has lost it.

• He could 

  have lost it.

• He lost it

A friend walked past you in the street and didn't stop to talk. She looked upset.

She could not have seen you. She must have had something on her mind.

Your brother is supposed to drive you to the airport. He's already 20 minutes late.

• His car must have broken down.

• He might be at home

• He doesn't remember

• He may not have remembered.

When I got home today, my husband was moping and sulking and didn't want to talk to me!

What is the formula to speculate about the past?

What is the difference between must, could, may and might?

Speculate about something (your own example)

Situation + speculation