The past form of "must"

Question

I am quite puzzled over this usage of the word "must":

"Members of his team said that he must forget everything he knew about swimming"

(This sentence is taken from English grammar textbook "Perspectives":

I was told that it was okay to say that, but this Wikipedia table on modal verbs says that the modal verb "must" doesn't have its past-tense form (unlike such modal verbs like, for example, "will" and "can" that have their clear past-tense forms "would" and "could"):

If so, then, as far as I understand, such past-tense sentense like "They told him 10 years ago that he must forget everything" must be grammatically incorrect (unlike some present-tense sentence like "Today I am telling you that you must forget it!").

What am I getting wrong here?

Answer 1

The sentence is grammatically incorrect for the reasons you guess, and the Wikipedia chart confirms. If I were marking a standardized writing exam, I would mark this as an error.

The past perfect "had learned" makes it unambiguous that "must forget" has a past meaning, not a present one, so it's incorrect.

THAT SAID, in unprepared speech, rather than in writing, I could imagine a less educated native speaker saying this, and a lot of people not even noticing the mistake. I hear this type of mistake from sports broadcasters all the time.

So, it's not going to cause communication problems, but it is wrong.

Answer 2

Must is a modal verb. Modal verbs do not change in form to make different tenses. Both your examples are fine, but they are using different verbs. 'Must' and 'had to' are not irregular forms of the same verb.

Of course, as auxiliary verbs, modals are used alongside another verb which most likely will have different forms. So, for example, if you say "you should go", retrospectively you might say "you should have gone".

'Must' is used differently to 'should'. We use 'should' for actions or responsibilities that we think are best for the concerned subject. We use 'must' things that are considered *compulsory or necessary. For this reason, we don't use 'must' to say that someone ought to have done something in the past - we would usually just say that they should have done it to fulfil their compulsory obligation. When we do use 'must' about something in the past it is to express disbelief that it wasn't done.

Answer 3

The passage speaks about Lewis Pugh, a swimmer, an idealist and a fervent environmentalist who swims the icy waters of lake Imja, 5,300 metres above sea level.

He failed the first time because he was swimming fast, therefore his crew urged him to change tactics and say at this altitude he must forget [present] everything he knew about swimming [past]; instead of swimming fast he must swim [present] as slowly as possible.

The original text uses “said" but if the author had written say it would have been easier to see why the modal verb must was kept and not substituted with had to. With had to the reader would have lost the sense of urgency, immediacy and drama in the team members' voice that the historical present has.

The historical present is used to narrate past events, it brings the action alive, it also adds drama to a story, which is why some call it the dramatic present. The deontic must employed in the original text is often uttered by authoritative figures such as parents, employers, coaches, and doctors but in this scenario the crew is not commanding Pugh to swim slowly; they are urging him to go against his instincts and everything he knows about swimming in perilous situations (swimming in the North Pole is given as an example). In this instance, must denotes deontic necessity.

In her blog, All Things Linguistics, the linguist Gretchen McCulloch explains

Notice that English is generally good at making distinctions between necessity and possibility but bad at making distinctions between epistemic and deontic, which must be cleared up via context. Some languages do make straightforward lexical distinctions between various flavours of modality like epistemic and deontic.

She provides an easy-to-use diagram which illustrates the two different modalities

In the diagram, the blue areas are epistemic modality (according to evidence, reasoning, or beliefs), the red areas are deontic modality (according to a set of rules or desires), the dark areas are necessity (in all possible worlds), the light areas are possibility (in at least one possible world).

Dark blue is epistemic necessity
“It must be raining outside (I can hear the rain).” In all worlds consistent with my beliefs, it is raining outside.

Dark red is deontic necessity
“It must rain this week (in order for the crops not to spoil)” In all worlds consistent with my desires, it rains this week.

Answer 4

It is perfectly reasonable and common for a sentence to mix verb tenses, if the actions in the sentence happen at different times.

A trivial example would be, "Yesterday I read a book and tomorrow I will watch a movie." The reading happened yesterday and so is past tense. The watching won't happen until tomorrow and so is future tense.

A slightly more complex example: "Yesterday I decided that I will go on vacation next Thursday." The deciding happened yesterday and so is past tense. But the trip itself is not until next Thursday and so is future tense.

"Must" doesn't make sense in past tense. "Must" makes a statement into a command, "You must do this". You can't command someone to do something in the past (assuming he doesn't have a time machine). You can only command someone to do something in the future.

So, "Members of his team said to him ten years ago that he must forget everything he had learned in school" makes perfect sense. The saying happened ten years ago and so is past tense. But what they said was a command of something he must do in the then-future. They didn't tell him that he must forget these things the day before they told him to forget. That would be logically impossible. They said he must forget sometime after they instructed him.

Consider, "Members ... said to him ten years ago that he will forget ..." The saying is ten years ago. The forgetting is in the future.

Side note: The future from when they said it, not necessarily from today. Like if I said, "In 1942 Winston Churchill said, 'We will win this war'", the saying is in the past. The winning is in the future from when he said it. By now that is also past, but it was future when he said it, so he used the future tense.