Die Festplatte enthält die Sicherungskopie.

Because both nouns are feminine singular.

• die Festplatte = feminine singular
• die Sicherungskopie = feminine singular

In German, the feminine singular article is:

• die in the nominative case
• die in the accusative case

So even though one noun is the subject and the other is the direct object, the article looks the same in both places.

A comparison with a masculine noun makes this easier to see:

• der Computer (nominative)
• den Computer (accusative)

With feminine nouns, you do not see that change.

In this sentence, you mainly know from normal word order:

• Die Festplatte = subject
• enthält = verb
• die Sicherungskopie = direct object

So the pattern is the standard subject + verb + object.

There is a small complication: because both noun phrases use die, the case marking does not clearly show the difference here. So German speakers rely on:

1. word order
2. context
3. common sense / meaning

That is very normal in German. Not every sentence is fully disambiguated by the articles alone.

It is in the accusative case because it is the direct object of enthalten.

The verb enthalten normally takes an accusative object:

• Die Datei enthält ein Bild.
• Der Ordner enthält viele Dokumente.
• Die Festplatte enthält die Sicherungskopie.

So if you learn the verb, it helps to remember:

• enthalten + accusative

Because the subject is die Festplatte, which is third person singular: it contains.

The infinitive is enthalten. Its present tense forms are:

• ich enthalte
• du enthältst
• er/sie/es enthält
• wir enthalten
• ihr enthaltet
• sie/Sie enthalten

So enthält is the correct form for er/sie/es and for any singular noun like die Festplatte.

Because enthalten is a strong verb with a stem-vowel change in the present tense.

It changes from a to ä in the du and er/sie/es forms:

• du enthältst
• er/sie/es enthält

This is similar to other strong verbs:

• fahren → du fährst, er fährt
• tragen → du trägst, er trägt
• halten → du hältst, er hält

Since enthalten is built from halten, it follows the same vowel-change pattern.

Because in German, all nouns are capitalized.

So in this sentence:

• Festplatte is capitalized because it is a noun
• Sicherungskopie is capitalized because it is a noun

This is one of the most noticeable spelling rules in German, and it applies everywhere, not just at the beginning of a sentence.

Yes. German very often combines words into one longer noun.

Festplatte

This is a compound noun. The final part is Platte, and that is why the whole noun is feminine:

• die Platte → die Festplatte

Sicherungskopie

This is also a compound noun. The final part is Kopie, so the whole noun is also feminine:

• die Kopie → die Sicherungskopie

A very useful rule for German compounds is:

• the last element determines the gender

The plurals are:

• die Festplatte → die Festplatten
• die Sicherungskopie → die Sicherungskopien

So you could say:

• Die Festplatten enthalten die Sicherungskopien.

Notice:

• Festplatte adds -n
• Sicherungskopie changes to Sicherungskopien

This sentence shows the most neutral order for a main clause:

• subject + verb + object

But the deeper rule in German is that the finite verb must usually be in second position in a main clause.

So German can move other elements for emphasis, as long as the verb stays second.

Example:

• Die Festplatte enthält die Sicherungskopie.
• Heute enthält die Festplatte die Sicherungskopie nicht mehr.

In your sentence, the basic word order is the most natural and straightforward one for a simple statement.

A simple learner-friendly pronunciation guide would be:

• Die Festplatte enthält die Sicherungskopie.
• roughly: dee FEST-plat-uh ent-HELT dee ZI-kher-oongs-ko-PEE

A few points:

• die sounds like dee
• ä in enthält is somewhat like the vowel in British air, but shorter
• ch in Sicherung is the soft German ch sound, not like English k
• the stress is usually:
  • FESTplatte
  • entHÄLT
  • SicherungskoPIE

If you want to sound natural, the main stress in the sentence usually falls on Festplatte and Sicherungskopie.

People very often say Backup in everyday German, especially in tech contexts.

So alongside die Sicherungskopie, you may also hear:

• das Backup

Both are used, but they are slightly different in style:

• Sicherungskopie sounds more formal or technical-German
• Backup sounds more everyday and very common in IT language

So depending on context, Germans might say either one.