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 'Must' as deduction and obligation
Michael Jackson

Elias Libor from the Czech Republic asks:

I study translating and interpreting. In one of our language lessons my teacher read a sentence from the (American) magazine ‘Time?

'The blaze......had to have been very strong? and said that was incorrect as it should have been must have been' instead. However, all the Americans I know told me this construction sounded perfectly natural to them.


Roger replies:

It is the case that the grammar of American English at times is slightly different from the grammar of British English, but where there are differences, most grammar reference books point out any alternative versions. For example, they might list examples of some of the past tense differences as follows:

British English
American English
He woke her with a kiss.
He waked her with a kiss.
They dived into the water.
They dove into the water.
I quitted the job after five days.
I quit the job after five days.
He burnt all her letters.
He burned all her letters.
The priest wetted the baby's head.
The priest wet the baby's head.
The cellar smelt of rotting apples.
The cellar smelled of rotting apples.

However, I have checked with all the grammar reference books that I have to hand and in none of them is had to have been listed as an alternative to must have been. It may have been used for emphasis.


Usually, had to indicates the past tense form of must when must = obligation. Study the following:

  • 'You?b>ll have to get up at five o?clock if you want to be in Birmingham by seven.'

  • 'I must remember to renew my car insurance at the end of the month.'

  • 'You mustn't park your car on the double yellow lines on the roads in Britain.'

  • 'She had to pay a fine of ?0 when she was caught speeding on the motorway.'
Note that if we use won't/don't/didn't have to as the negative of must, then we are expressing the absence of obligation or necessity and in this respect it is similar in use to needn't or don't/didn't need to. Compare the following:
  • 'You won't ever have to wear braces around your teeth again.' ( You needn't ever wear braces around your teeth again.)

  • 'You don't have to come, if you don't want to.' (You don't need to come.)

  • 'I didn't have to attend the January meeting, so I went to see Jane instead.' (I didn't need to attend the January meeting, so I went to see Jane instead.)

  • 'The following month, I had to present a paper, so my attendance was essential.

must as deduction

This usage of must is quite different. As in the Time article, we are registering that we are not absolutely sure about something, but are guessing or assuming that it has happened, will happen or is the case. In this sense, must have is the past tense form of must. Study the following:

  • 'It must be at least five weeks since we last met.'

  • 'You must be Helen. My mother has told me so much about you.'

  • 'We must have taken the wrong turning. We should be there by now.'

  • 'They must have missed their train. Otherwise they would be here by now.'

Note that the negative of must be or must have is can't be or can't/couldn't have been.

Here, again, we are making an assumption about something. Study the following:

  • 'She can't be very happy with her husband away on that oil rig all the time.'

  • 'It can't be lunch time already. That clock must be wrong.'

  • 'It couldn't have been Charlie who answered the phone because he's in hospital.'

  • 'I can't find Fifi anywhere. She can't have got out through the window. The opening is too small.