Für den Fall, dass der Wartungsvertrag endet, gilt die Wartungspauschale nicht.

Yes. Für den Fall, dass is a very common fixed expression meaning something like:

• in the event that
• in case
• should it happen that

It is slightly formal and is especially common in written, legal, or business German.

So in this sentence, Für den Fall, dass der Wartungsvertrag endet introduces the condition under which the rest of the sentence applies.

Because the preposition für always takes the accusative case.

The noun is der Fall in the dictionary form, but after für it becomes:

• der Fall → nominative
• den Fall → accusative

So:

• für den Fall

This has nothing special to do with the rest of the sentence; it is simply because für + accusative is required.

Because dass introduces a subordinate clause, and in German subordinate clauses are separated by a comma.

Here the subordinate clause is:

• dass der Wartungsvertrag endet

So the comma is mandatory:

• Für den Fall, dass der Wartungsvertrag endet, ...

German uses commas with this kind of clause much more consistently than English does.

Because dass introduces a subordinate clause, and in German the conjugated verb typically goes to the end of a subordinate clause.

Compare:

• Main clause: Der Wartungsvertrag endet.
• Subordinate clause: ..., dass der Wartungsvertrag endet.

That final verb position is one of the most important word-order patterns in German.

This is because German main clauses follow the verb-second rule.

The whole introductory phrase

• Für den Fall, dass der Wartungsvertrag endet

counts as the first element of the sentence. That means the finite verb of the main clause must come next, in second position:

• Für den Fall, dass der Wartungsvertrag endet, gilt die Wartungspauschale nicht.

If you started directly with the subject, you would get the more basic order:

• Die Wartungspauschale gilt nicht, für den Fall, dass der Wartungsvertrag endet.

But that version sounds less natural here. German often puts the condition first, and then the verb must immediately follow.

Because the subject of the main clause is singular:

• die Wartungspauschale

Even though die can look plural, here it is singular because the noun is die Pauschale.

So the verb must also be singular:

• die Wartungspauschale gilt

If it were plural, you would have:

• die Wartungspauschalen gelten

The subject of the main clause is:

• die Wartungspauschale

You can tell because it is the thing that gilt nicht.

The subordinate clause has its own subject:

• der Wartungsvertrag

So the sentence contains two clauses, each with its own subject and verb:

1. dass der Wartungsvertrag endet

   • subject: der Wartungsvertrag
   • verb: endet

2. gilt die Wartungspauschale nicht

   • subject: die Wartungspauschale
   • verb: gilt

Here gelten means something like:

• to apply
• to be valid
• to be in effect

So Die Wartungspauschale gilt nicht means the maintenance flat fee does not apply / is not valid.

That is close to ist nicht gültig, but gelten is often more idiomatic in formal or contractual language.

Compare:

• Die Regel gilt nicht. = The rule does not apply.
• Der Gutschein ist nicht gültig. = The voucher is not valid.

In your sentence, gilt nicht sounds very natural for contracts, rules, fees, and conditions.

Here nicht negates the predicate gilt:

• Die Wartungspauschale gilt nicht.
• The maintenance fee does not apply.

Its position is normal because nicht often comes toward the end of the clause, especially when it negates the whole verbal idea rather than just one specific word.

If you moved nicht, the emphasis could change. In this sentence, the usual neutral position is exactly where it is.

Yes, often you could.

Possible alternatives:

• Wenn der Wartungsvertrag endet, gilt die Wartungspauschale nicht.
• Falls der Wartungsvertrag endet, gilt die Wartungspauschale nicht.

Differences:

• wenn = neutral if/when
• falls = in case / if it happens that
• für den Fall, dass = more formal, often used in contracts or official writing

So in a business or legal context, Für den Fall, dass ... fits very well.

Yes. German very often forms long nouns by combining smaller words.

• Wartung = maintenance

• Vertrag = contract
  → Wartungsvertrag = maintenance contract

• Wartung = maintenance
• Pauschale = flat fee / fixed charge
  → Wartungspauschale = maintenance flat fee

A useful thing to remember is that the last part of a German compound noun determines the grammatical gender:

• der Vertrag → der Wartungsvertrag
• die Pauschale → die Wartungspauschale

Yes, it sounds formal and businesslike.

Clues include:

• Für den Fall, dass instead of simpler wenn
• vocabulary like Wartungsvertrag and Wartungspauschale
• gilt nicht, which is typical in rules, policies, and contracts

In everyday speech, a speaker might choose something simpler, such as:

• Wenn der Wartungsvertrag endet, gilt die Wartungspauschale nicht mehr.

But for official documents, the original sentence is perfectly natural.

Good question. Nicht mehr would mean no longer.

So:

• gilt nicht = does not apply
• gilt nicht mehr = no longer applies

The original sentence simply states non-applicability in that case. Adding mehr would emphasize that it used to apply before, but stops applying once the contract ends.

Depending on context, both could be possible, but they are not exactly the same. The original version is a bit more neutral and legalistic.