Opsigelsesfristen er en måned.

Because Danish, like English, often makes compound nouns, but Danish does this even more consistently and usually writes them as one word.

So:

• opsigelse = termination / notice
• frist = deadline / period / time limit

Together, opsigelsesfrist means notice period.

Then -en is added at the end to make it definite:

• opsigelsesfrist = a notice period
• opsigelsesfristen = the notice period

So the whole word is built like this:

• opsigelse + s + frist + en

The -s- is a linking sound/letter used in many Danish compounds. It does not mean possession here.

So this is not the same as English 's.

In opsigelsesfrist:

• opsigelse
  • s
    • frist

That s simply helps connect the parts of the compound in a natural way. Danish does this quite often, and learners usually just have to get used to which compounds use it.

Because in Danish, the definite article is usually added to the end of the noun.

Compare:

• en frist = a deadline / a period
• fristen = the deadline / the period

Since opsigelsesfristen already ends in -en, it already means the notice period.

So Danish does not need a separate the here.

Because en måned means a month, and here it is used as a predicate noun after er.

The structure is:

• Opsigelsesfristen = the notice period
• er = is
• en måned = a month

So the sentence literally works like:

• The notice period is a month.

That is why the second noun is indefinite:

• en måned = a month

If you said måneden, that would mean the month, which would change the meaning.

Because the sentence says the notice period is one month.

In Danish:

• en måned = one month / a month
• to måneder = two months

So singular is correct here.

English can sometimes say a month or one month, and Danish works similarly with en måned.

Er is the present tense of at være (to be).

So:

• jeg er = I am
• du er = you are
• det er = it is

Here, er links the subject and the description:

• Opsigelsesfristen er en måned.
• The notice period is one month.

It is the normal verb to use in this type of sentence.

Måned is a common gender noun in Danish, so it takes en in the singular indefinite form:

• en måned = a month

Its definite form is:

• måneden = the month

This is worth learning because Danish nouns are mainly either:

• common gender → en
• neuter gender → et

So here you must say en måned, not et måned.

The compound noun opsigelsesfrist takes its gender from the last part of the compound, which is frist.

• en frist = a deadline / time limit
• therefore en opsigelsesfrist = a notice period
• and the definite form becomes opsigelsesfristen

This is a very useful rule in Danish compounds: the final noun usually decides the gender.

This is standard Danish word order for a simple statement:

• Subject + verb + complement

So:

• Opsigelsesfristen = subject
• er = verb
• en måned = complement

This is completely normal for Danish main clauses.

A rough learner-friendly pronunciation might be:

• op-see-yel-ses-fris-ten er en mo-neth

A few useful notes:

• g in opsigelse is not pronounced like a hard English g
• d in måned is soft
• unstressed er and en are often pronounced very lightly in natural speech

You do not need perfect pronunciation right away, but it helps to hear the sentence spoken by native speakers because Danish pronunciation is often less phonetic than the spelling suggests.

Yes, but Opsigelsesfristen er en måned is a very natural and clear sentence.

You might also see related wording such as:

• Der er en opsigelsesfrist på en måned.
  = There is a notice period of one month.

That version uses a different structure, but the meaning is very close.

So the original sentence is not unusual at all—it is a normal, idiomatic way to say it.