Kattelemmen åpnes etter sosialiseringen.

Because kattelemmen is the definite singular form of kattelem, meaning the cat flap.

• en kattelem = a cat flap
• kattelemmen = the cat flap

Norwegian often adds the definite article to the end of the noun instead of using a separate word like English the.

The base word is kattelem. When it becomes definite singular, it takes the ending -en, but the spelling changes to -mmen:

• kattelem → kattelemmen

This happens because the final consonant is often doubled in writing when the noun has a short stressed vowel before it. You do not need to think about this as a separate ending; just learn kattelemmen as the correct definite form.

Åpnes is the present passive form of å åpne (to open).

So:

• å åpne = to open
• åpner = opens / is opening
• åpnes = is opened

In this sentence, åpnes most naturally means is opened.

They are very close in meaning.

• Kattelemmen åpnes etter sosialiseringen.
• Kattelemmen blir åpnet etter sosialiseringen.

Both can mean The cat flap is opened after the socialization.

The difference is mostly style:

• -s passive (åpnes) is compact and common in formal writing, instructions, notices, and procedural language.
• blir + past participle (blir åpnet) is often a little more explicit and can sound more natural in everyday speech.

Sosialiseringen is the definite singular form:

• en sosialisering = a socialization
• sosialiseringen = the socialization

The sentence is referring to a specific socialization process already understood from the context. That is why Norwegian uses the definite form.

Because etter is a preposition meaning after, and here it is followed by a noun phrase:

• etter sosialiseringen = after the socialization

If you want to follow after with a whole clause, Norwegian often uses etter at:

• etter at katten er sosialisert = after the cat has been socialized

So:

• etter + noun
• etter at + clause

It is grammatical, but it sounds somewhat formal and a little context-dependent.

It works best in a procedural or instructional context, for example if someone is describing when a cat flap should be unlocked or activated.

In everyday speech, a Norwegian speaker might also say something like:

• Kattelemmen åpnes etter at katten er sosialisert.
• Man åpner kattelemmen etter sosialiseringen.

So the original sentence is fine, but it has a somewhat formal, written feel.

Yes. Norwegian often moves time expressions to the front. If that happens, the verb still stays in second position.

For example:

• Kattelemmen åpnes etter sosialiseringen.
• Etter sosialiseringen åpnes kattelemmen.

Both are correct. The second version puts more focus on after the socialization.

Because Norwegian very often forms compound nouns as a single word.

So:

• katt = cat
• lem = hatch / flap
• kattelem = cat flap

English sometimes writes similar expressions as two words, but Norwegian usually joins them into one compound.

The dictionary form is å åpne.

That is the infinitive, meaning to open.

From it, you get forms like:

• å åpne = to open
• åpner = opens
• åpnet = opened
• åpnes = is opened

So if you are looking the word up, you should search for å åpne, not åpnes.