Der Einzelverbindungsnachweis ist online verfügbar.

Because Einzelverbindungsnachweis is a masculine noun, and in this sentence it is the subject.
So German uses the nominative singular article der.

• der = nominative masculine
• The subject is the thing that is available

So the structure is:

• Der Einzelverbindungsnachweis = the subject
• ist = is
• online verfügbar = available online

It is in the nominative case.

A quick way to see that is:

• it is the subject of the sentence
• it comes with der
• it is the thing doing the being available

So in Der Einzelverbindungsnachweis ist online verfügbar, the nominative subject is der Einzelverbindungsnachweis.

Because German main clauses normally follow the verb-second rule.

That means the conjugated verb comes in the second position in the sentence.

Here:

1. Der Einzelverbindungsnachweis
2. ist
3. online verfügbar

So even though English also says is in the middle here, in German this is part of a general rule: the finite verb usually comes second in a main clause.

German often builds long nouns by combining smaller words.

Einzelverbindungsnachweis can be broken down roughly as:

• Einzel- = individual / itemized
• Verbindung = connection, often used in telecom contexts for a call or connection
• Nachweis = proof, record, statement, verification

So the whole compound means something like an itemized connection record or detailed call record, depending on context.

The most important part in a German compound is usually the last part, because it determines:

• the basic meaning category
• the grammatical gender

Here the last part is Nachweis, so the whole word is masculine: der Nachweis → der Einzelverbindungsnachweis.

In German compound nouns, the last element determines the gender.

Here the last part is Nachweis, and Nachweis is masculine:

• der Nachweis

So the full compound is also masculine:

• der Einzelverbindungsnachweis

This is a very useful rule for long German nouns.

Verfügbar is an adjective meaning available.

In this sentence it comes after sein and works as a predicate adjective:

• ist verfügbar = is available

When an adjective comes after verbs like sein, werden, or bleiben, it usually does not take adjective endings.

So:

• Der Nachweis ist verfügbar.
• not Der Nachweis ist verfügbarer or anything like that

Here online functions like an adverbial modifier or part of the expression online verfügbar.

It tells you in what way or where/how something is available:

• online verfügbar = available online

A few useful points:

• online is borrowed from English
• it usually does not change form
• it often combines naturally with adjectives like verfügbar, buchbar, abrufbar

So online verfügbar is a very common German phrase.

Because online verfügbar is a very natural and common word combination in German.

It works almost like a set phrase:

• online verfügbar = available online
• online buchbar = bookable online
• online abrufbar = accessible online

Other word orders may be possible in special contexts, but ist online verfügbar is the standard, neutral phrasing.

It is understandable, but it sounds less natural in normal German.

The usual order is:

• ist online verfügbar

That is the idiomatic way to say it.

Putting online at the end can sound marked or unusual unless you are emphasizing online very strongly.

Because verfügbar is not being used directly before a noun.

Compare:

• der verfügbare Nachweis = the available record
  → here the adjective comes before the noun, so it gets an ending

• der Nachweis ist verfügbar = the record is available
  → here it comes after ist, so it gets no ending

That is the normal difference between:

• attributive adjectives: before nouns, with endings
• predicate adjectives: after sein and similar verbs, no endings

Yes. Then both the article and the verb change:

• Die Einzelverbindungsnachweise sind online verfügbar.

Changes:

• der → die
• ist → sind
• Einzelverbindungsnachweis → Einzelverbindungsnachweise

So the pattern is exactly what you would expect for a plural subject.

It is fairly formal and typical of billing, customer account, or telecommunications language.

The word Einzelverbindungsnachweis is especially common in:

• phone contracts
• mobile provider websites
• invoices
• account settings

In everyday speech, people might use shorter or simpler wording depending on context, but this sentence itself is perfectly normal in official or service-related German.