Hun bor i femte etasje.

Å bo is the infinitive, meaning to live / to reside.
In a normal present-tense sentence, Norwegian uses the present form bor:

• å bo = to live
• bor = live / lives

So Hun bor ... means She lives ...

Hun means she.

It is only capitalized here because it is the first word in the sentence. In the middle of a sentence, it would normally be written hun, not Hun.

No. Bo means to live somewhere / reside.

If you mean to be alive, Norwegian usually uses leve.

So:

• Hun bor i Oslo. = She lives in Oslo.
• Hun lever fortsatt. = She is still alive.

This is just a difference between the two languages.

In Norwegian, you normally say i femte etasje. Even though English uses on with floors, Norwegian usually uses i in this expression.

So the natural Norwegian pattern is:

• i første etasje
• i andre etasje
• i femte etasje

For standard Norwegian, i femte etasje is the normal and safest choice.

A learner should use i, not på, in this kind of sentence.

Because femte is an ordinal number: fifth.

• fem = five
• femte = fifth

When talking about floors, dates, rankings, and similar things, Norwegian uses ordinal numbers:

• første etasje = first floor
• andre etasje = second floor
• femte etasje = fifth floor

Norwegian often uses this kind of bare expression after a preposition when talking about location in a floor:

• i første etasje
• i tredje etasje
• i femte etasje

This is the normal, neutral way to say it.

You can also hear or read i den femte etasjen, but that sounds more specific or more marked, as if you are emphasizing that particular floor. For everyday use, i femte etasje is the most natural choice.

For the same reason: Norwegian often uses the noun without the definite ending in this fixed type of expression.

• etasje = floor / story
• etasjen = the floor

In ordinary location phrases with ordinal numbers, Norwegian usually prefers:

• i femte etasje

rather than:

• i den femte etasjen

Both can be possible in context, but the first one is the standard everyday form.

Etasje is a common-gender noun.

Its basic forms are:

• en etasje = a floor
• etasjen = the floor
• etasjer = floors
• etasjene = the floors

In this sentence, it appears in the expression i femte etasje.

Not always, depending on which variety of English you speak.

In Norwegian, første etasje is usually the ground/street level. That matches American usage more closely than British usage.

So femte etasje is:

• usually fifth floor in American English
• often fourth floor in British English terms

So the grammar of the Norwegian sentence stays the same, but the most natural English translation may depend on your dialect of English.

The sentence follows normal Norwegian main-clause word order:

• Hun = subject
• bor = verb
• i femte etasje = place expression

So it is:

Subject + Verb + Place

This is very common in simple Norwegian statements.