Nach dem Tarifwechsel buche ich mehr Datenvolumen zu.

Because nach in the sense of after takes the dative case in German.

• der Tarifwechsel = the plan/tariff change
• dative singular becomes dem Tarifwechsel

So:

• nach dem Tarifwechsel = after the tariff/plan change

This is a fixed grammar point: nach + dative when it means after.

Tarifwechsel is a compound noun:

• Tarif = tariff, rate, plan
• Wechsel = change, switch

So Tarifwechsel means a change of plan or switch to a different tariff/package, especially in contexts like mobile phone, internet, or utility contracts.

A natural English translation in this context is often plan change or switching plans.

Because the full verb is zubuchen, which is a separable verb.

In a normal main clause, separable verbs split apart:

• Ich buche Datenvolumen zu.
• base verb: zubuchen
• conjugated part: buche
• prefix: zu at the end

So the sentence structure is normal for a separable verb.

Not quite. In this context, zubuchen means something like:

• to add on
• to purchase additionally
• to top up with
• to add an extra package

This is common in telecom or contract language.
So mehr Datenvolumen zubuchen means to add more data allowance to an existing plan.

That is more specific than plain buchen, which often just means to book/reserve something.

Because German main clauses follow the verb-second rule.

The finite verb must come in the second position. If you start the sentence with a time phrase like Nach dem Tarifwechsel, that whole phrase takes the first position, so the verb comes next:

• Nach dem Tarifwechsel | buche | ich | mehr Datenvolumen zu.

So this is completely normal German word order.

Compare:

• Ich buche nach dem Tarifwechsel mehr Datenvolumen zu.
• Nach dem Tarifwechsel buche ich mehr Datenvolumen zu.

Both are possible, but the second version emphasizes the time frame more.

It is the accusative object of the verb.

The person is doing the action of adding/book­ing more data volume, so that noun phrase is the direct object.

However, you do not see a special ending here because:

• Datenvolumen is neuter singular
• it is used without an article
• the form looks the same in nominative and accusative

So grammatically it is accusative, even though the form does not change.

Because Datenvolumen is being used like a mass noun here, similar to water, money, or time in English.

German often uses no article with this kind of noun when talking about an unspecified amount:

• mehr Datenvolumen = more data allowance
• mehr Zeit = more time
• mehr Geld = more money

So the sentence is not talking about a specific, already identified data package, but simply more data volume in general.

Here it is singular.

Even though it refers to a quantity of data, Datenvolumen is usually treated as a single concept, like:

• bandwidth
• storage
• traffic
• data allowance

So:

• mehr Datenvolumen = more data allowance/data volume

You would not normally use a plural here in this meaning.

Because they are nouns, and in German all nouns are capitalized.

So:

• Tarifwechsel = noun
• Datenvolumen = noun

This is a basic spelling rule in German and one of the first things English speakers notice.

Primarily it is a time expression: after the plan change.

It tells you when the speaker adds more data volume.
It does not automatically mean because of the plan change, although in real life that idea may be implied by the context.

So grammatically, the phrase is temporal first of all.

Yes. For example:

• Ich buche nach dem Tarifwechsel mehr Datenvolumen zu.

That is also correct.

The version with Nach dem Tarifwechsel first puts more focus on the timing. German often moves time, manner, or place expressions to the front for emphasis, while still keeping the conjugated verb in second position.