Hun hadde på seg et belte og en skjorte med korte ermer da hun begynte å male.

Because ha på seg is a fixed expression meaning to wear or to have on.

• ha = have
• på = on
• seg = oneself / herself / himself, depending on the subject

So Hun hadde på seg ... means She was wearing ...

If you only said Hun hadde et belte, it would usually mean She had a belt, not necessarily that she was wearing it.

Because Norwegian nouns have grammatical gender.

• belte is a neuter noun, so it takes et
• skjorte is a common-gender noun, so it takes en

So:

• et belte = a belt
• en skjorte = a shirt

This is something you usually have to learn with each noun.

It means with short sleeves.

Breaking it down:

• med = with
• korte = short
• ermer = sleeves

So en skjorte med korte ermer is a shirt with short sleeves, or more naturally in English, a short-sleeved shirt.

Because ermer is plural.

In Norwegian, adjectives usually take -e in the plural:

• kort erme = short sleeve
• korte ermer = short sleeves

So korte agrees with the plural noun ermer.

Because Norwegian often uses a bare plural in phrases like this.

So med korte ermer literally means with short sleeves, not with the short sleeves.

This is very natural in Norwegian when describing clothing, features, or general characteristics.

It describes only en skjorte.

So the sentence means she was wearing:

• a belt
• a shirt with short sleeves

It would not normally be understood as a belt with short sleeves and a shirt. The meaning is guided by common sense as well as sentence structure.

Here da means when, referring to a specific event in the past.

• da = when, at that time, in a past event
• når = when, usually for repeated events, habits, or general time reference

So:

• da hun begynte å male = when she started painting

Because this is about one particular past moment, da is the correct choice.

Because begynne is followed by an infinitive with å.

• begynne å gjøre noe = to begin to do something

So:

• begynte å male = started to paint

This is very common in Norwegian:

• begynte å lese = started to read
• begynte å løpe = started to run

Here male means to paint.

So begynte å male means started painting.

A useful warning for learners:

• male = paint
• måle = measure

Those are different verbs, and the vowel makes the difference.

Because da hun begynte å male is its own clause, and that clause needs its own subject.

So the sentence has:

• main clause: Hun hadde på seg et belte og en skjorte med korte ermer
• subordinate clause: da hun begynte å male

The second clause still needs hun because Norwegian does not normally leave the subject out there.

No. Here hadde is just the simple past of ha.

That can confuse English speakers, because hadde often looks like had had in English. But in this sentence:

• hun hadde på seg = she was wearing / she had on

If you wanted a true past perfect idea, Norwegian would usually need something like hadde hatt på seg.

So this sentence is simply describing what she was wearing at the time she started painting.

Yes. That is also possible.

Norwegian often allows the parts of ha på seg to be split up:

• Hun hadde på seg et belte
• Hun hadde et belte på seg

Both are natural and mean roughly the same thing.

In your sentence, hadde på seg comes before the clothing items, which is a very common and clear way to say it.