Learn Hardcore German: Ich speichere die Datei auf dem Computer.

Datei (“file”) is a feminine noun in German, so its definite article is die. When you put a feminine noun in the accusative (direct-object) position, the article die stays die (only masculine der would change to den).

What case is dem Computer in, and why is it not accusative?

dem Computer is dative. The preposition auf is a two-way preposition (Wechselpräposition) that takes dative when showing a static location (you’re saving “on” the computer) and accusative when showing movement “onto” something.

Why do we use auf instead of in with Computer?

In German we say auf dem Computer to mean “on the computer” (like a surface or device). Saying in dem Computer would sound like you’re literally inside the machine’s housing, which isn’t what you want when talking about saving files.

Why is speichern conjugated as speichere here?

To match the subject ich (I), you take the verb stem speicher- and add the 1st-person-singular ending -e, giving ich speichere. Unlike some German verbs, speichern has no stem vowel change in the present tense.

Why is speichere placed immediately after ich?

German main clauses follow the verb-second (V2) rule: the finite (conjugated) verb must occupy the second position. Here Ich is first, so speichere comes right after, before the rest of the sentence.

Could I move auf dem Computer before die Datei?

Yes. If you start with the prepositional phrase for emphasis, you’d say:
Auf dem Computer speichere ich die Datei.
That shifts focus onto where you’re saving the file rather than what you’re saving.

How would I say “I’m saving the file on my computer”?

Use the dative form of the possessive pronoun mein. For masculine nouns in dative, mein becomes meinem:
Ich speichere die Datei auf meinem Computer.

Is there a difference between speichern and sichern?

Yes. speichern means “to save” (store a file), whereas sichern often means “to back up” or “to secure.” You’d say Ich sichere die Datei when you’re making a backup, for example to a server or external drive.

Can I drop ich and just say Speichere die Datei auf dem Computer?

Dropping ich in a declarative sentence isn’t standard and would sound like an imperative (“Save the file on the computer!”). In statements you normally include ich: Ich speichere ….

Why isn’t there any extra object marker or “to” in German, like English “save the file to the computer”?

German uses prepositions instead of extra objects for locations. The combination auf dem Computer already covers the meaning “to/on the computer,” so you don’t need an additional object marker.