«have»/»had» usados para la deducción

Es un uso Americano, a veces escuchado en el Reino Unido.

have/had se emplea principalmente con to be:

• There’s a tall grey bird fishing in the river. – It has to be/must be a heron. 

had + to be expresa el sentimiento de certeza del hablante en el pasado:

• There was a knock on the door. It had to be Tom. (She was sure it was Tom.)

had + to be es también una alternativa a must + infinitivo de perfecto:

• I wonder who took the money. – It had to be Tom/It must have been Tom. He’s the only who was there.  

Para evitar la confusión, se aconseja al estudiante apegarse a las formas must.