Ich werfe einen Stein ins Wasser.

the water

Breakdown of Ich werfe einen Stein ins Wasser.

• ich: I
• in: into
• das: the; (neuter, accusative)

Questions & Answers about Ich werfe einen Stein ins Wasser.

Why is it einen Stein and not ein Stein?

Because Stein is the direct object of the verb werfen, it has to be in the accusative case.

• Stein is masculine: der Stein (nominative).
• The masculine indefinite article (ein) changes in the accusative to einen.

So:

• Nominative (subject): Ein Stein liegt auf dem Boden. – A stone lies on the ground.
• Accusative (direct object): Ich werfe einen Stein. – I throw a stone.

In your sentence, einen Stein is what is being thrown, so it must be accusative.

How do I know that Stein must be in the accusative case here?

The accusative case is used for the direct object, i.e. the thing directly affected by the action.

Steps to see it:

1. Find the verb: werfe (from werfen – to throw).
2. Ask: What do I throw? → einen Stein.
3. That answer is the direct object → accusative.

So einen Stein is accusative because it answers what? after the verb and is directly acted on by werfen.

Why is it ins Wasser and not in das Wasser?