Learn Hardcore German: In der Stadt fahre ich oft mit dem Roller zur Arbeit, weil der Tunnel für Autos morgens voll ist. - In the city I often go to work by scooter because the tunnel for cars is full in the morning.

The phrase In der Stadt is placed at the beginning for emphasis (it’s a fronted adverbial of place).
That whole phrase counts as position 1.
The verb fahre must then be position 2.
The subject ich is pushed after the verb.

So:

Neutral order: Ich fahre in der Stadt oft mit dem Roller …
With fronted place phrase: In der Stadt fahre ich oft mit dem Roller …

In der Stadt ich fahre is wrong because that makes the verb third instead of second.

Why is it In der Stadt (dative) and not In die Stadt (accusative)?

The preposition in is a two-way preposition. It can take:

Accusative: when there is movement into or towards something.
Dative: when there is no movement, just describing a location.

In this sentence:

In der Stadt = in the city (within the city) → a static location → dative (der Stadt).
In die Stadt would mean into the city (movement toward the city), which is not the meaning here.

So In der Stadt fahre ich … = When I am in the city, I ride …
Not: I drive into the city …

Why is it fahre and not gehe for “go”?

German is more precise than English about how you move:

fahren = to go/travel/drive using a vehicle or wheels (car, bus, train, bike, scooter, etc.).
gehen = to go on foot, to walk.

Since a Roller (scooter) has wheels, you use fahren:

Ich fahre mit dem Roller zur Arbeit. = I go to work by scooter.
Ich gehe zur Arbeit. = I walk to work.

Why is it mit dem Roller and not mit den Roller or mit der Roller?

Two things are going on: the preposition and the noun’s gender.

mit always takes the dative case.
Roller (scooter) is masculine: der Roller.

The masculine singular forms of der are:

Nominative: der Roller
Accusative: den Roller
Dative: dem Roller

Because mit → dative, you must say mit dem Roller.

So:

mit dem Roller = by scooter
mit dem Auto = by car
mit dem Bus = by bus

What exactly does Roller mean here?

In this context, Roller usually means a scooter. Depending on region and context, it can be:

A motor scooter (like a Vespa).
Less commonly, sometimes a kick scooter.

It does not usually mean a skateboard or a bicycle.
For a bike, German uses das Fahrrad or das Rad.

Why is it zur Arbeit and not zu der Arbeit or in die Arbeit?

zur is a contraction of zu der:

zu
- der (feminine dative) → zur

Arbeit is feminine (die Arbeit). After zu, you need the dative:

zu der Arbeit → contracted → zur Arbeit

Using zu / zur Arbeit is the standard expression for going to work:

Ich fahre zur Arbeit. = I go to work.

in die Arbeit exists regionally (especially in parts of southern Germany / Austria), but zur Arbeit is more neutral and common in standard German.

Why is the verb at the end in …, weil der Tunnel für Autos morgens voll ist?

The conjunction weil introduces a subordinate clause, and in subordinate clauses the finite verb goes to the end.

Structure:

Main clause (verb in 2nd position):
In der Stadt fahre ich oft mit dem Roller zur Arbeit, …
Subordinate clause (verb at the end):
… weil der Tunnel für Autos morgens voll ist.

So:

Main clause: Der Tunnel ist voll. (verb = 2nd)
With weil: …, weil der Tunnel voll ist. (verb moves to the end)

Why is it der Tunnel and not den Tunnel in weil der Tunnel … voll ist?

In weil der Tunnel für Autos morgens voll ist, der Tunnel is the subject of the subordinate clause.

Masculine der forms:

Nominative (subject): der Tunnel
Accusative (direct object): den Tunnel

Here, Tunnel is not an object of any verb; it is what is full. So it must be nominative:

Der Tunnel ist voll. → subject in nominative.
In the weil-clause nothing changes about its role, so it stays der Tunnel.

Why do we say für Autos and not some dative form after für?

The preposition für always takes the accusative case in German.

The article for plural die Autos is:

Nominative plural: die Autos
Accusative plural: die Autos
Dative plural: den Autos

Because für requires accusative, you get:

für die Autos (often shortened in meaning to für Autos if the article isn’t needed).

In the sentence, Autos is used without an article, which is fine for plural. It still is accusative plural due to für, but the ending doesn’t show it here.

What is the difference between morgens and am Morgen?

Both relate to “morning”, but they’re used differently:

morgens = in the mornings, every morning, or usually in the morning
→ a habitual or repeated time.
am Morgen = in the morning (more like a specific morning or a specific day, especially when combined with other info).

In your sentence, you’re describing a regular situation:

… weil der Tunnel … morgens voll ist.
→ The tunnel is full every morning / in the mornings.

If you said:

… weil der Tunnel am Morgen voll ist.
it would sound more like talking about the morning of a particular day or a general fact about “the” morning time, but morgens is the standard for a regular, repeated situation here.

Could I say Ich fahre oft Roller zur Arbeit instead of mit dem Roller?

Normally, no. For many means of transport German allows:

Ich fahre Bus / Zug / Fahrrad. (without mit)

But this pattern is not so natural with Roller. Native speakers clearly prefer:

Ich fahre mit dem Roller zur Arbeit.

So:

Ich fahre Bus / Bahn / Zug. → OK.
Ich fahre Fahrrad. → Very common.
Ich fahre Roller. → Possible in some contexts, but sounds a bit unusual in a neutral sentence like this.
Ich fahre mit dem Roller … is the safest and most natural form.

Why is oft in the middle: fahre ich oft mit dem Roller? Where else could it go?

German adverbs like oft (often) are fairly flexible, but some positions sound more natural.

In the main clause:

In der Stadt fahre ich oft mit dem Roller zur Arbeit. (very natural)
In der Stadt fahre ich mit dem Roller oft zur Arbeit. (also possible, more emphasis on “often to work”).
In der Stadt fahre ich zur Arbeit oft mit dem Roller. (possible but less common, slightly clunky).

A common guideline is the time–manner–place order for adverbials:

morgens (time)
oft (frequency, often grouped with time)
mit dem Roller (manner)
in der Stadt / zur Arbeit (place/direction)

Your sentence already follows a natural rhythm; oft before mit dem Roller is a very typical choice.

Could I use denn instead of weil here? What’s the difference?

You could say:

In der Stadt fahre ich oft mit dem Roller zur Arbeit, denn der Tunnel für Autos ist morgens voll.

Differences:

Grammar / word order
- weil → subordinate clause → verb at the end:
  …, weil der Tunnel … voll ist.
- denn → coordinating conjunction → normal main-clause word order:
  …, denn der Tunnel … ist morgens voll.
Style / nuance
- weil → very common in spoken and written German, neutral.
- denn → a bit more written or formal, sounds like an explanatory “for/because”.

In everyday speech, weil is more frequent. Both are correct here, with the different word orders.