Sikkerhetshendelsen skjedde i går.

Because Norwegian, like German, often makes compound nouns by joining words together.

Here:

• sikkerhet = security
• hendelse = incident / event

So sikkerhetshendelse means security incident.

Then the definite ending is added:

• sikkerhetshendelse = a security incident
• sikkerhetshendelsen = the security incident

This is very normal in Norwegian.

The ending -en is the definite article attached to the noun.

So:

• en sikkerhetshendelse = a security incident
• sikkerhetshendelsen = the security incident

Unlike English, Norwegian usually puts the at the end of the noun instead of using a separate word.

In Norwegian, definiteness is usually built into the noun itself.

English:

• the incident

Norwegian:

• hendelsen

So in this sentence, sikkerhetshendelsen already includes the idea of the. You do not need a separate article before it.

Skjedde is the past tense of the verb å skje, which means to happen.

Forms:

• å skje = to happen
• skjer = happens / is happening
• skjedde = happened
• har skjedd = has happened

So Sikkerhetshendelsen skjedde i går is in the past because of skjedde.

No. Å skje is irregular.

That is why the past tense is skjedde, not something more predictable like skjete or skjete.

It is a verb worth memorizing:

• Det skjer ofte. = It happens often.
• Det skjedde i går. = It happened yesterday.
• Det har skjedd før. = It has happened before.

I går means yesterday.

It is a fixed time expression:

• i dag = today
• i morgen = tomorrow
• i går = yesterday

Even though i often means in, you should learn i går as a set phrase. You usually do not translate it word-for-word.

That is a very normal word order in Norwegian.

Basic order here:

• Sikkerhetshendelsen = subject
• skjedde = verb
• i går = time expression

So:

• Sikkerhetshendelsen skjedde i går.

You can also move the time expression to the front:

• I går skjedde sikkerhetshendelsen.

But then Norwegian requires verb-second word order, so the verb must come right after the first element:

• I går skjedde ... not
• I går sikkerhetshendelsen skjedde ...

Yes, if the context already makes it clear which incident you mean.

Compare:

• Hendelsen skjedde i går. = The incident happened yesterday.
• Sikkerhetshendelsen skjedde i går. = The security incident happened yesterday.

The longer version is more specific.

That s is a linking s often used in Norwegian compounds.

Here:

• sikkerhet
  • hendelse → sikkerhetshendelse

This extra s does not have a separate meaning by itself. It just helps form the compound in a natural way. English learners often notice this because English does not do it in quite the same way.

In Bokmål, hendelse is commonly treated as a feminine noun, but many feminine nouns can also take common-gender/masculine-style forms.

So you may see:

• en hendelse / hendelsen and sometimes
• ei hending exists as a different but related noun in some usage, though not the same standard form here

For this sentence, the important thing is simply:

• sikkerhetshendelse = indefinite singular
• sikkerhetshendelsen = definite singular

You would use skjer instead of skjedde:

• Sikkerhetshendelsen skjer i dag. = The security incident is happening today.

But semantically, incident often refers to something completed, so in real usage a present-tense version may sound less natural unless you are describing an ongoing event.

For past time, the original sentence is the natural one:

• Sikkerhetshendelsen skjedde i går.