Da der Bus heute nicht kommt, fahre ich mit dem Zug und steige ganz vorne in den ersten Waggon ein.

Because da introduces a subordinate clause. In German, the conjugated verb usually goes to the end of a subordinate clause.

So:

• Da der Bus heute nicht kommt = Since the bus isn’t coming today

The normal main-clause order would be:

• Der Bus kommt heute nicht.

But after da, it becomes:

• Da der Bus heute nicht kommt

This is one of the most important word-order patterns in German.

Here da means since or because.

So:

• Da der Bus heute nicht kommt ... = Since/Because the bus isn’t coming today ...

In many situations, da and weil are very similar. A rough difference is:

• weil = a straightforward because
• da = often since, sometimes slightly more formal or smoother when the reason is already known or taken as given

In everyday speech, learners often meet weil first, but da is very common in writing and careful speech.

Because der Bus is the subject of the clause.

In:

• Da der Bus heute nicht kommt

the bus is the thing doing the action of coming (or in this context, not coming / not running), so it stays in the nominative case:

• der Bus = nominative masculine singular

If it were a direct object, you might see den Bus, and if it followed a dative preposition or had another dative function, you might see dem Bus.

Yes, literally it is comes, but German often uses kommen with transport in ways that English might express differently.

Here der Bus kommt nicht can mean things like:

• the bus isn’t coming
• the bus won’t arrive
• sometimes even the bus isn’t running

German uses kommen naturally for scheduled arrivals and transport services, so this sounds normal.

Because the whole subordinate clause takes up the first position in the sentence, so the main clause must still keep the verb in second position.

German main clauses follow the verb-second rule. That means:

1. first position = one unit
2. second position = conjugated verb

Here the first position is the entire clause:

• Da der Bus heute nicht kommt

So the main clause begins:

• fahre ich mit dem Zug

not

• ich fahre mit dem Zug

This is a very common pattern:

• Wenn ich Zeit habe, gehe ich spazieren.
• Weil es regnet, bleibe ich zu Hause.

Because mit always takes the dative case.

So:

• mit dem Zug = by train
• mit dem Bus = by bus
• mit dem Auto = by car

The noun Zug is masculine, and its dative singular form with the definite article is:

• der Zug → dem Zug

Both are possible, but they are slightly different in feeling.

• mit dem Zug fahren = to travel/go by train
• den Zug nehmen = to take the train

In this sentence, fahre ich mit dem Zug focuses on the means of transport: I’m going by train.

That fits nicely after saying the bus is not coming.

The full verb is einsteigen, which means:

• to get in
• to board
• to get on (a train, bus, tram, etc.)

It is a separable verb.

In a main clause, the prefix separates and goes to the end:

• Ich steige ein.
• Ich steige in den Zug ein.

So in your sentence:

• ... und steige ganz vorne in den ersten Waggon ein.

the ein at the end belongs with steige.

In an infinitive or subordinate clause, it stays together:

• Ich will einsteigen.
• ..., weil ich in den ersten Waggon einsteige.

Because the subject ich is understood from the first main clause.

The sentence has two coordinated verbs with the same subject:

• fahre ich mit dem Zug
• und steige ganz vorne ... ein

German often leaves out the repeated subject when it is the same:

• Ich gehe nach Hause und koche Abendessen.

You could repeat ich, but it is usually unnecessary here.

Vorne means at the front or in front.

Ganz here is an intensifier, so:

• vorne = at the front
• ganz vorne = right at the front / all the way at the front

So the speaker is not just boarding near the front, but specifically at the very front end.

Because this sentence expresses movement into the carriage, not location inside it.

The preposition in is a two-way preposition:

• accusative for direction/movement into somewhere
• dative for location in somewhere

So:

• in den ersten Waggon = into the first carriage → movement → accusative
• im ersten Waggon = in the first carriage → location → dative

That is why den is used here.

Ersten is the adjective/ordinal form of erste (first), and it changes its ending because it follows den in the accusative masculine singular.

The noun is:

• der Waggon (masculine)

With in den ... Waggon, German needs:

• den ersten Waggon

This is normal adjective ending agreement after an article.

Waggon means a railway carriage/carriage car or train car.

In modern German, you may also often hear:

• Wagen
• Zugwagen in some contexts
• regionally and practically, people may simply say things like im ersten Wagen

Waggon is completely understandable and correct, though it can sound a bit more formal or traditional than some everyday alternatives.

German often places adverbials and complements before the separable prefix at the end.

Here the important structure is:

• steige ... ein

Everything else fits in between:

• steige ganz vorne in den ersten Waggon ein

A natural way to understand it is:

• steige = finite verb
• ganz vorne = where/at what part of the train
• in den ersten Waggon = into which carriage
• ein = separable prefix completing einsteigen

This sounds natural in German, even if the English order would be quite different.

Not in this sentence.

German da can mean different things depending on context:

• da = there
• da = since/because
• da = then / in that case in some contexts

Here, because it is followed by a full clause with the verb at the end, it clearly means since/because:

• Da der Bus heute nicht kommt ...

So this is not the location word there.