Die Rufnummernmitnahme ist kostenlos.

Die is the definite article for a feminine singular noun in the nominative case, and Rufnummernmitnahme is feminine.

A useful clue is the verb ist. Because ist is singular, die cannot be plural here. If it were plural, the sentence would need sind.

So here:

• die Rufnummernmitnahme = the number porting / the phone-number transfer
• feminine singular, nominative

It is in the nominative case because it is the subject of the sentence.

The basic structure is:

• Die Rufnummernmitnahme = subject
• ist = verb
• kostenlos = predicate adjective

So the sentence is built like The number transfer is free.

German very often combines nouns into a single compound noun.

Here, Rufnummernmitnahme is made from:

• Rufnummer = phone number
• Mitnahme = taking along, transfer, carry-over

German writes this as one word, not as separate words.

A very important rule: in German compounds, the last part usually determines the:

• core meaning
• grammatical gender

Since Mitnahme is feminine, the whole compound Rufnummernmitnahme is feminine too.

That -n- is a linking element used in some German compounds.

So instead of combining the parts as Rufnummermitnahme, German uses Rufnummernmitnahme.

These linking sounds are very common in German compounds, for example:

• Arbeit
  • Zimmer → Arbeitszimmer
• Liebe
  • Brief → Liebesbrief

Unfortunately, they are not always fully predictable, so learners often just have to learn the compound as a whole.

Yes, kostenlos is an adjective.

It has no ending here because it is used after the verb sein in a predicate position. Predicate adjectives in German do not take adjective endings.

So:

• Die Rufnummernmitnahme ist kostenlos.
  Here kostenlos stays plain.

But before a noun, it would take an ending:

• eine kostenlose Rufnummernmitnahme

That is the same basic difference as:

• The service is free
• a free service

In a normal German main clause, the finite verb usually comes in second position.

Here the order is:

• Die Rufnummernmitnahme = first position
• ist = second position
• kostenlos = the rest

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

For example:

• Die Rufnummernmitnahme ist kostenlos.
• Heute ist die Rufnummernmitnahme kostenlos.

In both sentences, the finite verb ist stays in second position.

Because it is a noun, and German capitalizes all nouns.

In this sentence:

• Rufnummernmitnahme is capitalized because it is a noun
• kostenlos is not capitalized because it is an adjective
• ist is not capitalized because it is a verb

Die is capitalized here only because it is the first word of the sentence.

Yes. Mitnahme comes from the verb mitnehmen, which literally means to take along.

So this noun has the idea of taking something with you. In the context of phone service, it means taking your existing phone number with you when changing providers.

This is a common pattern in German:

• verb: mitnehmen
• noun: die Mitnahme

Yes. This is a very direct parallel to English.

German:

• Die Rufnummernmitnahme ist kostenlos.

English structure:

• The number transfer is free.

So the pattern is:

• subject
• sein = to be
• adjective

That makes this sentence grammatically quite straightforward once you recognize the long compound noun.

Yes, kostenfrei would also be correct.

So you could say:

• Die Rufnummernmitnahme ist kostenlos.
• Die Rufnummernmitnahme ist kostenfrei.

Both mean free of charge. In many contexts, kostenfrei can sound a little more formal or business-like, while kostenlos is very common everyday German.

Then the adjective would need an ending.

Compare:

• Die Rufnummernmitnahme ist kostenlos.
  Predicate adjective, so kostenlos has no ending.

• die kostenlose Rufnummernmitnahme
  Attributive adjective before a noun, so it becomes kostenlose

This is a very common contrast in German, and it is worth learning early:

• after sein: no adjective ending
• before a noun: adjective ending needed