Learn Hardcore German: Der Schlüssel liegt neben dem Computer.

Because Schlüssel is a masculine noun in German. In a simple declarative sentence the subject (nominative case) masculine singular definite article is der.

Why is dem used before Computer instead of den?

The preposition neben takes the dative case when you specify a static location (answering Wo? “where?”). The dative masculine singular article is dem, so neben dem Computer.

When does neben ever take the accusative case?

When you express movement toward a position (answering Wohin? “where to?”). For example:
• Ich lege den Schlüssel neben den Computer.
Here neben governs accusative (den Computer) because it describes the action of placing.

What’s the difference between liegen and legen?

Liegen is an intransitive verb meaning “to lie” or “to be located” (no direct object).
Legen is a transitive verb meaning “to lay” or “to put something down” (requires a direct object).
In your sentence the key already lies there, so you use liegt (3rd person singular of liegen).

Could you also say Der Schlüssel ist neben dem Computer?

Yes. Der Schlüssel ist neben dem Computer is grammatically correct and means the same. However, German often uses location verbs (liegen, stehen, sitzen) for objects in a fixed position.

Why is Computer capitalized?

All German nouns are capitalized, regardless of where they appear in a sentence. Computer is a noun, so it begins with a capital letter.

How would you ask “Where is the key?” in German?

You would say: Wo liegt der Schlüssel?
– Wo = where
– liegt = lies/is located
– subject follows the verb in a question.

Can you change the word order to start with the location?

Yes. German is a V2 (verb‑second) language. You can place Neben dem Computer first and keep the finite verb in second position:
Neben dem Computer liegt der Schlüssel.