Hvis der ikke kommer nogen taxa, går vi over broen til fods.

Here der is a dummy subject, similar to English there in expressions like there is or there comes. It does not point to a place.

Danish often uses der when introducing the existence or arrival of something:

• Der kommer en bus = a bus is coming
• Der er et problem = there is a problem

So der kommer nogen taxa means something like any taxi arrives/turns up.

Because Hvis der ikke kommer nogen taxa is a subordinate clause introduced by hvis.

In Danish, sentence adverbs like ikke usually come:

• after the finite verb in a main clause
• before the finite verb in a subordinate clause

Compare:

• Main clause: Der kommer ikke nogen taxa
• Subordinate clause: hvis der ikke kommer nogen taxa

This is a very common word-order pattern in Danish.

Danish often uses the present tense for future meaning when the context makes the time clear.

So kommer here can mean comes or will come, and går vi can mean we walk or we will walk. The condition introduced by hvis already makes it clear that the sentence is about a future situation.

This is similar to English in the if-clause:

• If no taxi comes, ...

English also normally uses present tense there, not will come.

This is because Danish main clauses follow the V2 rule: the finite verb usually comes in the second position.

In this sentence, the whole hvis-clause comes first:

Hvis der ikke kommer nogen taxa

That first clause counts as position 1, so the finite verb of the main clause must come next:

går vi over broen til fods

So the order becomes:

1. fronted element: Hvis der ikke kommer nogen taxa
2. finite verb: går
3. subject: vi

If you started directly with the main clause, you would say:

Vi går over broen til fods

In a negative context, nogen usually means any.

So ikke kommer nogen taxa means no taxi comes / there does not come any taxi / no taxi turns up.

Danish very often uses ikke + nogen where English might use no. So for a learner, it is useful to remember:

• nogen in negatives and questions often means any
• ingen means no / none

With a sentence like this, ikke ... nogen is a very natural choice.

Yes. Taxa is singular.

• en taxa = a taxi
• taxaen = the taxi
• taxaer = taxis
• taxaerne = the taxis

So in this sentence, nogen taxa refers to any taxi, not several taxis.

Because Danish usually adds the definite article to the end of the noun.

• bro = bridge
• broen = the bridge

So English the bridge becomes Danish broen.

If there is an adjective, Danish normally also uses a separate article:

• den gamle bro = the old bridge

But here there is no adjective, so broen by itself is enough.

In this sentence, the most natural meaning is across the bridge.

Danish over can mean both over and across, depending on context. With movement involving a bridge, English usually says across the bridge, even though Danish uses over.

So går vi over broen means we walk across the bridge.

Til fods is a fixed expression meaning on foot.

It is idiomatic, so it is best learned as a whole phrase. Literally, it is not translated word-for-word in normal English. If you try to translate each part separately, it will sound less natural than simply learning:

• til fods = on foot

So går vi over broen til fods means we go across the bridge on foot.

Yes. You can also say:

Vi går over broen til fods, hvis der ikke kommer nogen taxa.

That is grammatically correct and means the same thing.

The difference is mainly focus:

• Hvis der ikke kommer nogen taxa, går vi over broen til fods puts the condition first
• Vi går over broen til fods, hvis der ikke kommer nogen taxa starts with the action instead

When the sentence starts with the hvis-clause, the main clause shows inversion: går vi. When the sentence starts with the main clause, the normal order is vi går.