Help with Germanp please?

Question

Hi,
I'm just getting a little confused!
If in English you say that you 'must not' do something, do you use the modal mussen or durfen?

Also, what would be the missing words in this sentence if you're using "sollen; perfect tense"
Wir ______ das fenster nicht aufmachen________

thanks very much for your help

very sorry, spelling mistake, I mean German

Answer 1

Yes, it's confusing both ways.
While Engliah changes words when putting may and must to opposites german does not

I may - I must not
Ich darf - ich darf nicht
I must - I need not
Ich muss - ich muss nicht

If in doubt find the positive form e.g. must not - may and use the German translation with nicht

Your sentence would then be:
Wir haben das Fenster nicht aufmachen sollen.

This is a special form (Ersatzinfinitiv - replacement infinitive)as the "Infinitiv" replaces the "Partizip Perfekt" sollen instead of "gesollt" this is always done with
dürfen, können, mögen, müssen, sollen, wollen and brauchen if it stands in a composited time (e.g. past tense) after an "Infinitiv" (here "aufmachen").

In general the past perfect (Perfekt) is the perfect form used in spoken German while in written German it is the "Imperfekt also Präteritum or Mitvergangenheit" = past tense (simple)

If you don't find ü,ä,ö on your keyboard use ue, ae, oe (Ue, Ae, Oe) instead.

@leef: You wrote:
The modal verbs, apart from "haben" und "sein", are the only verbs that are colloquially used in the simple past instead of the perfect tense.
I cannot fully agree with you - here in Austria (and most likely in southern Germany as well) we would not use simple past for the modal verbs. To stick with your sentence it would rather be:
Ich hab' gestern nicht fortgeh'n dürfen and not: Ich durfte gestern nicht fortgehen.

Answer 2

One of the difficult bits is "(I)must not..." in English which translates into German as "(ich) darf nicht...", whereas the German "ich muss nicht..." translates into English as "I need not..."

The previous answer to the sentence with the missing verb forms is correct, but in German it is NOT colloquially used.
The modal verbs, apart from "haben" und "sein", are the only verbs that are colloquially used in the simple past instead of the perfect tense.
That way cumbersome forms like "Ich habe gestern nicht ausgehen können" are avoided, in favour of " Ich konnte gestern nicht ausgehen".

Answer 3

you must not smoke

= du darfst nicht rauchen

wir haben das Fenster nicht aufmachen sollen