Cómo se usa Must para suponer algo

 Cómo se usa Must para suponer algo

Cuando queremos suponer sobre algo que  ha pasado ya,  usamos siempre la misma estructura para todas las personas gramaticales:
"must have" más el participio perfecto del verbo en cuestión.

You've just seen a pair of Napoleon's trousers in a museum.
You conclude:

• He must have been short.

A friend tells you of the night he spent in the snow in the
mountains. You say to him:

• You must have been freezing.

Another friend tells you of how he saw a shark whilst swimming.
You say to him:

• You must have been terrified. 

Your wife tells you how her grandfather used to eat eight
churros a day. You conclude:

• He must have been fat.

Your friends should have arrived at 7:00 p.m. It's now 9:15
p.m. You conclude:

• They must have got lost.  

Para la forma negativa,  se recurre al verbo "can" y la negación "not"
La estructura es "can't have" más el participio perfecto del verbo.
Lo que estamos expresando en este caso es nuestra de que no nos creemos lo que pensamos.

You've just been told that Real Aranjuez has beaten
Valencia. You say in disbelief:

• They can't have beaten Valencia. 

Your husband tells you he can't find his car.
You say in disbelief:

• You can't have lost your car.

After five minutes your daughter tells you she has finished
her homework. You say in disbelief:

• You can't have finished your homework yet.

Your wife, exaggeration personified, tells you of her awful
experience in the supermarket. You say in disbelief:

• It can't have been that bad!

A colleague tells you how your boss insulted him.You say in
disbelief:

• He can't have said that!  

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