På den ene siden opptjeningsperioden; på den andre siden foreldrepengeperioden.

Because this is an elliptical expression: Norwegian often leaves out words that are understood from the context.

A full version could be something like:

• På den ene siden har vi opptjeningsperioden; på den andre siden har vi foreldrepengeperioden.
• På den ene siden er det opptjeningsperioden; på den andre siden er det foreldrepengeperioden.

But in headings, summaries, comparisons, charts, and official texts, Norwegian often drops the verb and keeps only the key elements.

So this sentence is not wrong; it is just compressed.

It is a fixed comparison pattern meaning:

• on the one hand ... on the other hand ...
• sometimes more literally on one side ... on the other side ...

In this sentence, it sets up a contrast or a two-part division between:

• opptjeningsperioden
• foreldrepengeperioden

So the structure is very common when comparing two perspectives, two categories, or two stages.

In this expression, Norwegian normally uses the definite article:

• på den ene siden
• på den andre siden

This is the standard fixed form.

Here:

• den = the
• ene = one
• andre = other/second

So literally it is something like:

• on the one side
• on the other side

English usually drops the in the idiomatic translation on the one hand / on the other hand, but Norwegian keeps it.

No. Here siden is the noun side in the definite singular form:

• side = side
• siden = the side

So:

• på den ene siden = on the one side
• på den andre siden = on the other side

This is completely different from siden meaning since, which is a conjunction or adverb in other contexts.

Because the expression is built that way: på den ene siden / på den andre siden.

The noun is definite because it refers to the one side and the other side of a contrast. Norwegian commonly uses the definite form in this set phrase.

So the structure is:

• på
  • den
    • adjective/ordinal-like word + siden

Examples:

• på den ene siden
• på den andre siden

Because Norwegian forms compound nouns very freely, much more than English does.

So:

• opptjening = earning / accrual
• periode = period
• opptjeningsperioden = the accrual/qualifying period

and:

• foreldrepenger = parental benefit / parental pay
• periode = period
• foreldrepengeperioden = the parental-benefit period

In Norwegian, these compounds are normally written as single words, not as separate words.

The ending -en marks the definite singular form for many masculine nouns.

So:

• en periode = a period
• perioden = the period

In these compounds:

• opptjeningsperioden = the qualifying/accrual period
• foreldrepengeperioden = the parental-benefit period

So both nouns are definite: they refer to specific periods, not just any periods.

Because the article is already built into the noun.

In Norwegian, the definite article is often attached to the end of the noun:

• en periode = a period
• perioden = the period

So you do not add a separate the-word before it in this sentence. The definiteness is already expressed by -en.

It is natural in the right context, especially in:

• headings
• summaries
• bullet points
• official or bureaucratic writing
• diagrams or comparisons

In ordinary full-sentence prose, many speakers would more often say something like:

• På den ene siden har vi opptjeningsperioden, på den andre siden foreldrepengeperioden.
• Det er forskjell på opptjeningsperioden og foreldrepengeperioden.

So yes, it can sound incomplete if you see it alone, but it is still natural as a compressed contrast structure.

The semicolon separates two closely related parts of one contrast:

• På den ene siden opptjeningsperioden
• på den andre siden foreldrepengeperioden

A semicolon works well here because the two parts are parallel and strongly connected.

A comma might also appear in some contexts, but the semicolon gives a clearer sense of two balanced halves.

You can understand it that way, but in more natural English you would usually say:

• On the one hand, the qualifying period; on the other hand, the parental-benefit period.
• or, if you make it into a full sentence: On the one hand, there is the qualifying period; on the other hand, there is the parental-benefit period.

So a literal translation is possible for understanding the Norwegian structure, but a more idiomatic English version usually sounds better.

Yes, mostly. This is a set expression, so learners should treat it as a chunk:

• på den ene siden
• på den andre siden

Changing the order would usually sound wrong or at least unnatural.

It is best to memorize the whole pattern as one unit.

Not if you want this standard contrast expression.

The normal idiomatic pattern is:

• på den ene siden ... på den andre siden ...

Using på en side or på en annen side would not sound like the usual fixed contrast formula. It may be grammatically possible in some literal physical context, but it is not the standard expression for balancing two ideas.

Because in Norwegian, common nouns are not capitalized the way they are in German.

So you write:

• opptjeningsperioden
• foreldrepengeperioden

with lowercase letters unless they begin the sentence.

Only proper names and sentence-initial words get capitals in normal writing.