Learn Hardcore Norwegian: Søsteren min er ingeniør i en moderne fabrikk. - My sister is an engineer in a modern factory.

So in the sentence, Søsteren min literally means “the sister my”, which is how Norwegian normally says “my sister” when the possessive comes after the noun.

What is the difference between søsteren min and min søster?

Both can mean “my sister”, but there are two differences:

Form
- søsteren min
  - definite noun + postposed possessive
  - literally: the sister my
- min søster
  - preposed possessive + indefinite noun
  - literally: my sister
Use and nuance
- søsteren min is the most neutral, everyday way to say my sister.
- min søster can sound a bit more emphatic or contrastive, especially in speech, like:
  - Min søster er ingeniør, ikke broren min.
    My sister is an engineer, not my brother.

Grammatically, both are correct; søsteren min is simply more common in neutral statements like this one.

Why is it søsteren min with min, and not søsteren mi?

In Bokmål, søster is historically feminine, but it is very common to treat many feminine nouns as common gender (masculine).

Masculine/common gender possessive (singular): min
Feminine possessive (singular): mi
Neuter possessive (singular): mitt
Plural (all genders): mine

So you can see:

søsteren min (common gender style – very standard)
søstera mi (clearly feminine style – also correct, but sounds more dialectal/informal)

Your sentence uses the very standard, neutral pattern: søsteren min.

What is the difference between søster, søsteren, and søstra?

They are all forms of søster (sister), but with different definiteness and style:

søster – a sister (indefinite)
- en søster = a sister
søsteren – the sister (definite, standard Bokmål, common gender)
- søsteren min = my sister
søstra – the sister (definite, clearly feminine form)
- more colloquial/dialectal Bokmål, often with ei søster – søstra mi

In standard written Bokmål, en søster – søsteren min is the safest choice.

Why is there no article before ingeniør? In English we say “an engineer”.

In Norwegian, after the verb er (is/are), you normally do not use an article with professions, nationalities, religions, and similar roles:

Hun er lærer. – She is (a) teacher.
Han er student. – He is (a) student.
Søsteren min er ingeniør. – My sister is (an) engineer.

So Søsteren min er ingeniør is the natural, standard way to say it.
Adding en (er en ingeniør) is possible in some contexts, but then there is usually a nuance or extra information (see next question).

Can I say Søsteren min er en ingeniør?

You can, but it sounds less neutral and is used in more specific situations. For example:

To identify what kind of person she is in a more general sense:
- Han er en ingeniør, ikke en kunstner.
  He is an engineer, not an artist.
When you add a descriptive adjective:
- Søsteren min er en dyktig ingeniør.
  My sister is a skilled engineer.

If you’re just giving her profession in a simple way, Søsteren min er ingeniør is better and more natural.

Why is it i en moderne fabrikk and not something like på en moderne fabrikk?

Both i and på can be used with workplaces, but they give slightly different impressions:

i en moderne fabrikk
Literally in a modern factory.
Emphasises being inside the physical building/space.
på en moderne fabrikk
Literally at/on a modern factory.
More about being employed at that factory as a workplace.

In many contexts, both are acceptable. Your sentence with i works well if you imagine her working in the factory environment. If you want to stress the job location more generally, på en moderne fabrikk would also be fine.

Why is the word order i en moderne fabrikk and not i en fabrikk moderne?

In Norwegian noun phrases, the normal order is:

preposition + article + adjective + noun

So:

i en moderne fabrikk
= in a modern factory

Putting the adjective after the noun (i en fabrikk moderne) is not normal Norwegian word order. To place the adjective later, you would need a relative clause:

i en fabrikk som er moderne
= in a factory that is modern

But the short form i en moderne fabrikk is the normal, natural choice.

Why doesn’t the adjective moderne change form? Shouldn’t it agree with fabrikk?

Adjectives in Norwegian usually agree with the noun in gender/number, but moderne is one of the adjectives that already ends in -e and does not change in the indefinite:

en moderne fabrikk – a modern factory (masc./common)
et moderne hus – a modern house (neuter)
moderne fabrikker – modern factories (plural)

So moderne looks the same for all genders and for plural here.
(If the noun phrase were definite and had an adjective, you would also add a separate definite article: den moderne fabrikken = the modern factory. But moderne itself still stays the same.)

Why is it en moderne fabrikk and not et moderne fabrikk or ei moderne fabrikk?

Because fabrikk is a masculine/common gender noun in Bokmål.

Masculine/common gender: en fabrikk, fabrikken
Neuter: et hus, huset
(Optional) Feminine forms exist for some nouns, but fabrikk is normally treated as masculine.

So the correct indefinite form is:

en fabrikk → en moderne fabrikk

How would I say “My sisters are engineers in modern factories” using this sentence as a model?

You need plurals for søster, ingeniør, and fabrikk:

søster → søstre (irregular plural), søstrene (definite plural)
ingeniør → ingeniører
fabrikk → fabrikker

Two natural versions:

With preposed possessive (indefinite plural noun):
- Mine søstre er ingeniører i moderne fabrikker.
- My sisters are engineers in modern factories.
With postposed possessive (definite plural noun):
- Søstrene mine er ingeniører i moderne fabrikker.
- Also My sisters are engineers in modern factories.

Both are correct; (2) feels slightly more specific (“the sisters of mine”).

What is the difference between Søsteren min er ingeniør and Søsteren min jobber som ingeniør?

Both can describe her job, but they focus on slightly different things:

Søsteren min er ingeniør.
States her profession/identity. It’s like saying “She is an engineer (by profession).”
Søsteren min jobber som ingeniør.
Literally: “My sister works as an engineer.”
Focuses more on what she currently does for work. It could, in some contexts, allow for the idea that she might also have some other background.

In everyday use, they usually mean the same thing, but the first is more about who she is professionally, the second more about what she does (her job).

Does søster in Norwegian mean specifically “older/younger sister”, or just “sister” in general?

Søster on its own just means “sister” in general, without saying whether she is older or younger.

If you need to be specific, you add another word:

storesøster – older/big sister
lillesøster – younger/little sister

So Søsteren min er ingeniør i en moderne fabrikk only tells us that it’s my sister, not whether she is older or younger.