Learn Hardcore German: Du darfst das Fenster keinesfalls in der Nacht offen lassen. - You must by no means leave the window open at night.

du kannst das Fenster nicht offen lassen – you cannot / are not able to leave the window open (focus on ability).
du sollst das Fenster nicht offen lassen – you should not leave the window open (a recommendation or instruction).
du darfst das Fenster nicht/keinesfalls offen lassen – you may not / must not leave the window open (a rule or prohibition).

Here, the idea is clearly “not allowed”, so dürfen is the natural verb.

What exactly does keinesfalls mean, and how strong is it?

keinesfalls literally comes from kein + -falls and means “by no means / under no circumstances / absolutely not”. It’s stronger and more emphatic than a simple nicht.

Rough strength comparison:

nicht – not
auf keinen Fall – under no circumstances
keinesfalls – by no means / absolutely under no circumstances (very firm and a bit formal)

So:

Du darfst das Fenster nicht offen lassen.
= You are not allowed to leave the window open.
Du darfst das Fenster auf keinen Fall offen lassen.
= You must absolutely not leave the window open.
Du darfst das Fenster keinesfalls offen lassen.
= Very similar to auf keinen Fall, slightly more compact and somewhat formal or emphatic.

In normal spoken German, many people might prefer auf keinen Fall; keinesfalls can sound a bit more written or serious.

Why is keinesfalls placed before in der Nacht? Could it go somewhere else?

German word order for adverbs and negation is quite flexible, but keinesfalls is a kind of sentence adverb / negation that usually stands relatively early in the middle field, before many other details.

Your sentence:

Du darfst das Fenster keinesfalls in der Nacht offen lassen.

This order is very natural: subject – verb – object – negation – time – infinitive.

Other possible placements:

Du darfst das Fenster in der Nacht keinesfalls offen lassen.
– Also correct. This can put a bit more emphasis on in der Nacht as the condition that is being forbidden.
Du darfst keinesfalls das Fenster in der Nacht offen lassen.
– Grammatically fine, but now the emphasis is more on das Fenster (not that window, absolutely not).

What you generally don’t do is put keinesfalls after the verb at the very end in a main clause:

✗ … offen lassen keinesfalls. – sounds wrong in standard German.

So yes, it can move somewhat, but it usually stays in the “middle” of the sentence, not at the beginning or the very end.

Why is it in der Nacht and not in die Nacht?

In der Nacht uses the dative case because it answers “Wann?” (When?), not “Wohin?” (Where to?).

With in, German uses:

Akkusativ (in die …) for movement into something (wohin?),
Dativ (in der …) for location or time (wo? / wann?).

Here, in der Nacht = at night / during the night, a time expression, so it takes dative:

in der Nacht – dative, feminine die Nacht → der Nacht

If you said in die Nacht, it would mean “into the night”, which is directional and doesn’t fit this meaning.

A very common alternative is simply:

nachts – at night / during the night

So you could also say:

Du darfst das Fenster nachts keinesfalls offen lassen.

Same meaning, slightly more natural and shorter in everyday speech.

Why is it das Fenster and not some other case like dem Fenster?

Das Fenster is the direct object of the verb lassen (“to leave (something) in a certain state”).

Pattern:

jemand lässt etwas offen
someone leaves something open

The “something” that is left in a state is in the accusative:

ich lasse das Fenster offen – I leave the window open.
das Fenster = accusative singular (neuter).

Declension of das Fenster (neuter, singular):

Nominative: das Fenster (subject)
Accusative: das Fenster (direct object – same form)
Dative: dem Fenster
Genitive: des Fensters

So dem Fenster would be dative, for example:

Ich stehe am Fenster. – I stand at the window.
Ich helfe dem Fenster – this would be grammatically possible but makes no sense (you don’t “help” a window).

In our sentence, das Fenster is the thing being left open → accusative object.

What is the role of lassen here, and how does offen lassen work?

Lassen is often used with an adjective or a past participle to mean “to leave something in a certain state”.

Pattern:

etwas + Adjektiv/Partizip + lassen
= to leave something + adjective/participle

Examples:

Ich lasse das Fenster offen. – I leave the window open.
Lass die Tür zu! – Leave the door closed!
Wir lassen das Licht an. – We leave the light on.

In your sentence:

das Fenster offen lassen
= to leave the window open

So offen is not a verb; it’s an adjective describing the state of the window, and lassen is the main lexical verb whose meaning is “leave (it like that)”.

With the modal verb dürfen, the main verb lassen goes to the end in the infinitive:

Du darfst das Fenster … offen lassen.

So the structure is:

du (subject)
darfst (modal verb)
das Fenster (object)
keinesfalls in der Nacht (adverbials)
offen lassen (infinitive phrase: “leave open”)

Why is it offen and not geöffnet?

Both offen and geöffnet can describe something that is open, but there are differences in use and nuance:

offen is the basic adjective “open” and is very common for everyday physical things:
- das Fenster ist offen – the window is open
- die Tür ist offen – the door is open
geöffnet is more like “opened / open (for business)”, often used for:
- shops, offices, opening hours:
  Das Geschäft ist geöffnet. – The store is open (for customers).
- a result state (has been opened):
  Die Flasche ist geöffnet. – The bottle is opened.

For “leave the window open”, offen lassen is the normal, idiomatic choice:

das Fenster offen lassen – leave the window open (completely standard)
das Fenster geöffnet lassen – grammatically okay, but sounds more formal/technical and less idiomatic in everyday speech.

How would this sentence look in a subordinate clause, for example after dass?

In a subordinate clause, the conjugated verb (here the modal darfst) normally goes to the end, after the other infinitives:

Main clause:

Du darfst das Fenster keinesfalls in der Nacht offen lassen.

Subordinate clause (for example after ich weiß, dass …):

Ich weiß, dass du das Fenster keinesfalls in der Nacht offen lassen darfst.

Word order:

dass (subordinating conjunction)
du (subject)
das Fenster (object)
keinesfalls in der Nacht (adverbials)
offen lassen darfst (verb group at the end: adjective + infinitive + finite modal)

So the double-verb structure lassen darfst is grouped at the very end, with the finite verb (darfst) last.

How would this change if I wanted to be formal and say Sie instead of du?

To address someone formally, you use Sie (capital S). You also need the corresponding verb form:

du darfst → Sie dürfen

So the full sentence formally:

Sie dürfen das Fenster keinesfalls in der Nacht offen lassen.
= You (formal) are absolutely not allowed to leave the window open at night.

Other changes in context:

Possessives: dein Fenster → Ihr Fenster (your window, formal)
Imperatives, pronouns etc. would also change, but in this sentence only du → Sie and darfst → dürfen are needed.

What is the difference between Du darfst das Fenster keinesfalls offen lassen and Du darfst das Fenster nicht offen lassen?

Both sentences express prohibition, but the strength and tone differ:

Du darfst das Fenster nicht offen lassen.
– Neutral prohibition: You are not allowed to leave the window open.
– Could be a simple rule or instruction.
Du darfst das Fenster keinesfalls offen lassen.
– Strong, emphatic prohibition: You must absolutely not, under any circumstances, leave the window open.
– Suggests that the consequences are serious or that the speaker is very strict.

So keinesfalls adds emotional weight and emphasis. If you just want to say “don’t do that” in a neutral way, nicht is usually enough.

Is dürfen here closer to English “may not” or to “must not”?

In this context, du darfst … keinesfalls is best translated as “must not” rather than “may not”.

Literal: “You are under no circumstances allowed to …”
Natural English: “You must not leave the window open at night.”

While dürfen in a positive sentence is usually “may / be allowed to”, a negative with strong emphasis (nicht, auf keinen Fall, keinesfalls) often corresponds to “must not / absolutely not allowed to” in English.

So conceptually:

Du darfst das Fenster nicht offen lassen.
~ You may not / must not leave the window open.
Du darfst das Fenster keinesfalls offen lassen.
~ You absolutely must not leave the window open.

Could I replace in der Nacht with nachts without changing the meaning?

Yes. nachts is a very common, natural adverb meaning “at night / during the night”.

So you can say:

Du darfst das Fenster keinesfalls nachts offen lassen.

The meaning is practically the same:

in der Nacht – at night, a bit more explicit and slightly more formal.
nachts – at night, short and very common in everyday speech.

Stylistically:

Rules, warnings, and official instructions might use in der Nacht.
Everyday spoken German often prefers nachts.

Both are correct and idiomatic.

Where does the stress fall when saying this sentence aloud, especially with keinesfalls?

A natural pronunciation in standard German might stress:

DU DARFST das FENster KEInesFALLS in der NACHT OFfen LASSEN.

Key points:

darfst is often stressed when you want to underline the (lack of) permission.
Fenster can be stressed to highlight what must not be left open.
keinesfalls almost always carries strong stress because it’s the emphatic negation.
in der Nacht can be additionally stressed if the time condition is important:
… keinesfalls IN der NACHT offen lassen (it might be okay during the day).
offen lassen is often slightly less stressed; it’s the action itself, but the emotional focus is usually on the negation and the condition.

In practice, a speaker might strongly stress keinesfalls and Nacht to sound particularly warning or serious.