Det var ikke før den andre uken at hun husket navnene på alle i pauserommet.

This is a very common Norwegian pattern meaning It was not until ... that ....

So:

• Det var ikke før den andre uken at hun husket ...
• = It was not until the second week that she remembered ...

It is used to emphasize when something finally happened.

Here, det is not referring to a specific thing. It is part of a fixed cleft-style structure:

• Det var ... at ...

In English, this is similar to sentences like:

• It was then that ...
• It was not until later that ...

So det is basically a formal/dummy subject in this pattern.

At is the conjunction that.

In this sentence, it introduces the clause:

• at hun husket navnene på alle i pauserommet

English often drops that, but Norwegian usually keeps at in this kind of structure.

So the sentence is built like this:

• Det var ikke før X at Y
• = It was not until X that Y

Because Norwegian normally uses double definiteness when a noun is definite and has an adjective or ordinal before it.

So you get:

• den = the
• andre = second
• uken = week + definite ending

Together:

• den andre uken = the second week

This is very typical Norwegian grammar.

That is exactly what double definiteness means.

In many cases, Norwegian marks definiteness twice:

1. with a separate article: den
2. with a definite ending on the noun: -en

So:

• uke = week
• uken = the week
• den andre uken = the second week

To an English speaker, this can feel repetitive, but it is normal Norwegian grammar.

Here it means second.

That said, andre can also mean other in some contexts. For example:

• den andre can mean the other one
• but in time expressions like den andre uken, it is naturally understood as the second week

So context decides the meaning.

Husket is the preterite (simple past) of å huske = to remember.

So:

• husker = remember / remembers
• husket = remembered

In this sentence, it tells us that the remembering happened at a specific time in the past.

Navnene is the definite plural form of navn.

• et navn = a name
• navn = names
• navnene = the names

The sentence is talking about the specific names of the people in the break room, not just names in general, so the definite form is used.

With navn, Norwegian often uses på where English would often use of or a possessive structure.

So:

• navnet på henne = her name / the name of her
• navnene på alle = everyone’s names / the names of everyone

This use of på is very natural in Norwegian.

Here alle means everyone.

Even though alle can also mean all, in this sentence it clearly refers to people:

• navnene på alle i pauserommet
• = the names of everyone in the break room

So alle is standing on its own as a pronoun.

Pauserommet means the break room.

It comes from:

• pauserom = break room
• pauserommet = the break room

The ending -et is the definite ending for this neuter noun.

Because the sentence refers to a specific break room, one that is already known from context.

So:

• i et pauserom = in a break room
• i pauserommet = in the break room

Using the definite form suggests this is a particular room relevant to the situation.

Yes. A simpler version would be:

• Hun husket ikke navnene på alle i pauserommet før den andre uken.

This means basically the same thing.

The original version:

• Det var ikke før den andre uken at ...

is a bit more emphatic, because it highlights when it finally happened.