Navigationen viser, at der er vejarbejde foran os, så vi bør dreje tidligere.

In Danish, the definite article is usually attached to the end of the noun.

• navigation = navigation
• navigationen = the navigation

So Navigationen viser ... means The navigation shows ...

This is very common in Danish:

• bil → bilen = the car
• vej → vejen = the road

Here Navigationen probably refers to the GPS or navigation system that is already known from the situation.

At here means that and introduces a subordinate clause.

So:

• Navigationen viser = The navigation shows
• at der er vejarbejde foran os = that there is roadwork ahead of us

This is very similar to English:

• He says that ...
• The app shows that ...

In everyday English, that is often omitted, but in Danish at is very commonly kept in sentences like this.

Der er is the normal Danish way to say there is / there are when introducing the existence of something.

So:

• der er vejarbejde = there is roadwork

This der does not really mean a physical place like there in over there. It is the standard existential construction.

Compare:

• Der er en bil udenfor. = There is a car outside.
• Der er vejarbejde foran os. = There is roadwork ahead of us.

Det er would usually mean it is, which is a different structure.

Because Danish usually writes compound nouns as a single word.

Here:

• vej = road
• arbejde = work
• vejarbejde = roadwork / road construction

This is a very important feature of Danish. Where English often uses two words, Danish often uses one:

• togstation = train station
• sommerferie = summer holiday
• arbejdsplads = workplace

So vejarbejde is completely normal Danish spelling.

Because Danish treats vejarbejde as a singular or mass noun.

So even though English often uses the plural-looking word roadworks, Danish uses:

• der er vejarbejde = there is roadwork

The verb is singular:

• er = is

This is one of those places where Danish and English package the idea differently. Danish is closer to English roadwork than to roadworks.

Because foran is a preposition, and after a preposition Danish uses an object pronoun, not a subject pronoun.

So:

• vi = we
• os = us

Therefore:

• foran os = in front of us / ahead of us

This is the same idea as in English:

• not in front of we
• but in front of us

Here så means so in the sense of therefore / as a result.

So the sentence is structured like this:

• Navigationen viser, at der er vejarbejde foran os = The navigation shows that there is roadwork ahead of us
• så vi bør dreje tidligere = so we should turn earlier

After så, this is still a main clause, and the normal word order is used:

• vi bør = we should

That is why it is så vi bør ..., not så bør vi ... in this version.

If you used an adverb like derfor instead, inversion would happen:

• Derfor bør vi dreje tidligere. = Therefore we should turn earlier.

Because bør is a modal verb, and modal verbs in Danish are followed directly by the infinitive without at.

So:

• vi bør dreje = we should turn

This works like:

• vi kan dreje = we can turn
• vi må dreje = we may / must turn
• vi skal dreje = we must / have to turn

You do not add at after these modal verbs.

Tidligere means earlier.

It is the comparative form, so it implies comparison with an expected time or point:

• dreje tidligere = turn earlier / turn sooner

That fits the meaning here: because of the roadwork, they should turn before they originally planned to.

Why not the others?

• tidligt = early
• tidligere = earlier
• før = before / earlier, but used differently depending on the structure

In this sentence, tidligere is the most natural way to express earlier than planned.

Here dreje means to turn, especially in navigation language.

So vi bør dreje tidligere means they should make their turn sooner than expected.

Danish also has svinge, which can also mean turn, especially in driving contexts. But dreje is very common in instructions from maps and GPS systems:

• Drej til højre = Turn right
• Drej til venstre = Turn left

So in this sentence, dreje is a very natural choice.