... und sie darf es nie erfahren. – Ist doch grammatisch auch Gegenwart.

Ja, aber der Rest des Satzes nicht, wie auch der Rest der Lektüre.

OK, Du hast einen Miniausschnitt gewählt. Vielleicht ist das auch eine allwissende Erzählperspektive nach dem Motto: Viele Jahre werden noch ins Land gehen ...

... und sie sollte es niemals erfahren.

Sofern ich mich an meinen Englischunterricht korrekt erinnere, sind die Formen von must in allen Zeiten gleich. Hier ist es eben past tense.

Das ist interessant. Im Deutschen ist die normale Erzählform die Vergangenheit und "... und sie darf es nie erfahren." wäre auf Deutsch ziemlich ungewöhnlich, wenn alles andere in der Vergangenheit ist. "... und sie durfte es auch nie erfahren." ist das Normale.

Würde mich auch interessieren, wie das auf Englisch wirkt.

Mehr Kontext: Julie und ihre Freunde haben den Tod eines kleinen Jungen verschuldet,indem sie ihn mit dem Auto anfuhren und Fahrerflucht begingen. Das war letzten Sommer und das "darf/durfte" die Mutter nie erfahren.Der Satz, den ich abgeschrieben habe ist aus Julies Perspektive geschrieben...
Danke für die Hilfe!

Das Problem ist, dass 'must' so recht keine Vergangenheitsform hat. Meist benutzt man deshalb 'had to':

I must help him now.
I had to help him yesterday.

Da 'She never had to know' aber hier auch nicht geht, bleibt nur 'must' - soweit meine laienhafte Erklärung. Es ist jedenfalls eine übliche Konstruktion.

Was erwartest du eigentlich? Musted? Das gibt es nicht!

Dirk, du bist ganz schön selbstbewusst! Lass dir das bloß von keinem nehmen!

Danke Gibson. Das macht Sinn. Im Deutschen ist "sie durfte es nie erfahren" nicht das "durfte" im Sinne eines in der Vergangenheit liegenden Verbotes (ich durfte nicht alleine in Urlaub fahren, ehe ich 18 war) sondern wie "man muss verhindern, dass...sonst" oder so ähnlich gemeint.

sind die Formen von must in allen Zeiten gleich.

Das stimmt eben so nicht. Du kannst weder sagen 'I must help him yesterday' noch 'I have must'. Ich halte die Frage für berechtigt; Hilfverben sind eh schwierig, und 'must' und 'should' für Deutsche doppelt.

Sorry, wenn meine eilig hingeschriebenen Antworten ein wenig arrogant rübergekommen sein sollten. Was ich meinte, ist: Eine Form musted gibt es nicht und eine Umschreibung mit had to ist hier nicht möglich. Also bleibt nur die Möglichkeit must. Und ich meine, mich aus Schulzeiten zu erinnern, dass must in bestimmten Situationen eben auch für die Vergangenheitsform stehen kann. Dies hier wäre genau so eine Situation.

Aber ein Muttersprachler kann das mit Sicherheit kompetenter erklären.

Im Past Tense muss es heißen:
"She was not to know."
Beispiele:
Yes I spoke to the rest of the family and they all agreed that she was not to know [that she had cancer].
http://cancerchat.cancerresearchuk.org/messag...

Well, his wife was long gone, but the father, it seemed, had been a double agent, and was also gone, but this Laura was not to know, ever.
books.google.com/books?isbn=1409080943

Das Problem ist, dass 'must' so recht keine Vergangenheitsform hat. (#7)

It’s a defective verb:
http://en.wikipedia.org/wiki/Defective_verb

Danke an alle. Habe echt was gelernt heute!

I agree that the form "must" is normal in certain situations where a preterite would normally be required.

I'm not sure exactly what the restrictions are, but I think it is common where you would normally expect the backshifted preterite, which is used principally - but not exclusively - with indirect speech (as in "it was agreed that this must never happen again").

I would tend to say that I understand it as a past tense in these circumstances (or perhaps I should say that I pragmatically infer the anteriority of the deontic necessity).

The problem with "was to" is that it is often ambiguous and can also be used to represent a past perspective of an intermediate event (time t -1 from the perspective of t -2 as reported at t 0): "she was not to know the truth until much later" could mean that she did not know the truth till much later (at a time that was then in the future but is now in the past).

"she was not to know the truth until much later"

Dieser Satz ist von seiner Aussage her anders als meine Beispielssätze in #13.
Hier wäre im Deutschen die Übersetzung mit "sollte nicht" zutreffend.
In meinen Sätzen ist die Bedeutung "durfte nicht" ersichtlich.

Das gilt auch wohl für die Form im Präsens (is not = darf nicht):

"Que sera, sera
Whatever will be, will be
The future's not ours to see."

Vielleicht liege ich ja daneben, aber für mich klingt
"The mother didn't know why Julie had changed so much since last year, and she must never know."
weder nach Gegenwart noch nach Vergangenheit, sondern nach Futur, etwa wie
"... und sie würde es nie erfahren dürfen."

"The mother didn't know why Julie had changed so much since last year, and she must never know."

I read she must never know as present tense or some sort of absolute tense or aside. Mother (in the past) didn't know something. She must never know!

Edit: I hadn't seen #18.

marker

I share your disquiet about 'must'. If I understand the context rightly, it means: '... and it was important/essential/imperative that she never know' (or, if you don't like the subjunctive: that she should never know).

'must' is shorter, and there is no equally short alternative; that is why it's been used here.

I was writing something else, but then saw #18 & #19 (and now #20)-- that's the way I'm reading the sentence, too. It's clearly not the past -- that would be something like "...and she was never to know."

edit

I find 'must' in the OP perfectly OK. I agree with escoville's paraphrase of it in #20:

it means: '... and it was important/essential/imperative that she never know' (or, if you don't like the subjunctive: that she should never know).

-- but I don't share escoville's disquiet about it. I take it as a past tense which just happens to have the same form as the present.

I think there is a form of implied indirect speech here: Julie, or someone else, is thinking, 'Mum must never know!' There's a subjective, even emotional element, which would not be there in, for example, 'was to'. I don't think 'was to' would work at all in the sentence.

As people have said, "must" does not have a past.

In my opinion, the standard way of expressing the past would be "she could never know". It does not mean "sie konnte nie wissen/erfahren"; it means "sie durfte nie wissen/erfahren".

Die Comprehensive Grammar schreibt dazu:

"Corresponding to must, which cannot normally be used in reference to past time outside indirect speech or indirect thought contexts, the past tense form had to can be used in main clauses. Compare:
I must confess her latest novels bore me.
I had to confess her latest novels bored me." (§4.61 note b, p. 232)

"Must, together with need (as auxiliary), ought to, and had better, has no present/past distinction. These verbs are therefore unchanged in indirect speech constructions, even where they refer to past time." (§4.60, p. 231)

Randolph Quirk et al. (1985): A Comprehensive Grammar of the English Language.

The mother did not know - and I don't want her to ever know.
I don't find the OP surprising, curious or wrong.

Re #25: I don't find the OP surprising, curious or wrong. 

I agree. I do think that one would more likely encounter "and she must never know" in writing than in spoken conversation--but it would be proper in any event.

I suppose the OP's example is actually ambiguous (i.e. it could also be understood as present, referring to the future from the narrator's point of view, but I would normally understand it as referring to a past necessity.

I think indirect speech covers most examples of "must" used where a preterite would be expected; however there are some cases where it is not clearly indirect speech but the sentences are not felt to be clearly incorrect (though there may be some "disquiet"):

"At the age of eighty, the time had come when he must leave the stage."
"He ate – when he must – the flesh of . . ."
"The moment had come when he must act."