Der Boden im Flur knackt, wenn ich nachts aufstehe.

Because der Boden is the subject of the sentence — it is the thing doing the action of knackt.

• der Boden = nominative
• den Boden = accusative
• dem Boden = dative

In Der Boden im Flur knackt, the floor is what creaks, so nominative is required.

im is a contraction of in dem, so it contains dative:

• in dem Flur → im Flur

Here, in describes a location and not movement toward somewhere, so German uses the dative.

Compare:

• im Flur = in the hallway / located in the hallway
• in den Flur = into the hallway / movement into it

Because im is the normal contraction of in dem. It is extremely common and usually sounds more natural than the full form.

So:

• im Flur = standard, natural
• in dem Flur = possible, but often more emphatic or stylistically heavier

knackt is the 3rd person singular present tense form of knacken.

Conjugation:

• ich knacke
• du knackst
• er/sie/es knackt

Since der Boden is singular, the verb is knackt.

Because wenn ich nachts aufstehe is a subordinate clause, and in German subordinate clauses are separated from the main clause by a comma.

Main clause:

• Der Boden im Flur knackt

Subordinate clause:

• wenn ich nachts aufstehe

This comma is mandatory in standard German.

Because wenn introduces a subordinate clause, and in German subordinate clauses the conjugated verb goes to the end.

So:

• Main clause: Ich stehe auf.
• Subordinate clause: wenn ich aufstehe

Since aufstehen is a separable verb, in a subordinate clause its parts stay together at the end:

• aufstehe
  not
• stehe auf

Because aufstehen is a separable verb.

In a main clause, separable verbs split:

• Ich stehe nachts auf.

In a subordinate clause, they do not split:

• ..., wenn ich nachts aufstehe.

That is one of the most important patterns with separable verbs.

This is a very common question.

• wenn = when/whenever for repeated events, general conditions, or future situations
• als = when for a single event in the past
• wann = when? in a question or indirect question

In this sentence, wenn is correct because it means something like whenever or when(ever) I get up at night.

Examples:

• Wenn ich nachts aufstehe, knackt der Boden.
  = Whenever I get up at night, the floor creaks.

• Als ich letzte Nacht aufstand, knackte der Boden.
  = When I got up last night, the floor creaked.

• Wann stehst du nachts auf?
  = When do you get up at night?

nachts is an adverb, meaning at night / during the night. It is very common and natural.

So:

• nachts = at night, during the night
• in der Nacht = in the night / during the night, also possible but less compact

In this sentence, nachts sounds very idiomatic.

It is an adverbial form historically related to the genitive, but for learners the most useful thing is simply:

• treat nachts as a fixed adverb meaning at night

Similar time expressions exist in German, for example:

• morgens = in the morning
• abends = in the evening
• tagsüber = during the day

So nachts is not functioning like a noun phrase here.

In a subordinate clause, German often keeps the subject early in the clause unless there is a reason to emphasize something else.

The neutral order here is:

• wenn ich nachts aufstehe

That gives:

1. subordinating conjunction: wenn
2. subject: ich
3. time adverb: nachts
4. verb at the end: aufstehe

You could change the order for emphasis in some contexts, but this version is the most neutral and natural.

Yes, absolutely. That is equally correct.

German allows either order:

• Der Boden im Flur knackt, wenn ich nachts aufstehe.
• Wenn ich nachts aufstehe, knackt der Boden im Flur.

When the subordinate clause comes first, the main clause still follows the verb-second rule, so you get:

• Wenn ich nachts aufstehe, knackt ...

Notice that knackt comes immediately after the comma.

Yes. German present tense often describes:

• habitual actions
• general truths
• repeated situations

So this sentence suggests a repeated or typical situation:

• whenever I get up at night, the floor creaks

It is not necessarily about one single specific night.

Yes. der Flur usually means:

• hallway
• corridor
• entry hall

The exact English translation depends on the house or building. In everyday German, it often refers to the indoor passage area connecting rooms.

Because nachts is not built like an English noun phrase such as at night. It functions as a single adverb.

So you do not analyze it as:

• the night

Instead, think of it as a time adverb:

• nachts = at night

That is why there is no separate article there.