Streken på kartet er rød.

Because streken is the definite form of strek.

In Norwegian, the equivalent of English the is often attached to the end of the noun:

• en strek = a line
• streken = the line

So Streken på kartet means the line on the map.

For the same reason: kartet is the definite form of kart.

• et kart = a map
• kartet = the map

So på kartet means on the map.

Because the sentence is talking about a specific line and a specific map.

• streken = the particular line being referred to
• kartet = the particular map being referred to

This works just like English The line on the map is red.

Because på kartet is a prepositional phrase that describes which line we mean.

Norwegian usually puts this kind of modifier after the noun:

• streken på kartet = the line on the map
• boka på bordet = the book on the table

This is very similar to English word order.

Because på usually means on, and a line is seen as being on the surface of the map.

So:

• på kartet = on the map

Using i kartet would usually suggest something more like in the map, which does not fit this idea as well.

Because rød agrees with streken, which is an en-noun in singular.

After er, Norwegian adjectives still agree with the noun:

• en strek → Streken er rød
• et hus → Huset er rødt
• flere streker → Strekene er røde

So here rød is correct because the subject is singular and an en-noun.

Because rød is not inside the noun phrase here; it comes after er as a predicate adjective.

Compare:

• den røde streken = the red line
  (adjective directly describing the noun)
• streken er rød = the line is red
  (adjective after is)

In Norwegian, when an adjective comes before a definite noun, you normally get double definiteness:

• den røde streken
• det røde huset
• de røde linjene

But in Streken på kartet er rød, the adjective is not part of the noun phrase, so there is no den.

Because this is a normal Norwegian main clause: the finite verb usually comes in the second position.

Here the first element is the whole subject:

• Streken på kartet = first element
• er = second element

That is standard Norwegian word order.

You can change the order for emphasis, for example:

• På kartet er streken rød

But the original sentence is the most neutral version.

• strek is an en-noun
• kart is an et-noun

That affects their forms:

• en strek → streken
• et kart → kartet

It also affects adjective agreement:

• streken er rød
• kartet er rødt

So knowing the gender helps you choose the right article, definite ending, and adjective form.

Not always, though they can overlap.

Strek often means a drawn line, mark, or stroke. It can feel very visual and concrete.

Linje is broader. It can mean a line too, but also things like:

• a route
• a line in a graph
• a line of text
• a policy or direction

In Streken på kartet er rød, strek suggests an actual visible line drawn on the map.

The hardest part for many English speakers is ø.

A simple guide:

• ø is a rounded front vowel that English does not really have
• your lips are rounded, but the tongue is placed more forward than for English o

Also, in many pronunciations of Norwegian, the final d in rød is silent or very weak. So it may sound roughly like a long rø.

So rød is often pronounced approximately like:

• røø

But the exact pronunciation varies somewhat by dialect.