Learn Hardcore German: Ich werfe einen Stein ins Wasser.

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:

Find the verb: werfe (from werfen – to throw).
Ask: What do I throw? → einen Stein.
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?

Ins is simply a contraction of in das:

in (preposition: in, into)
das (neuter accusative article for das Wasser)
→ ins

So:

Ich werfe einen Stein ins Wasser.
Ich werfe einen Stein in das Wasser.

Both are grammatically correct and mean the same. In normal spoken and written German, the contracted form ins Wasser is much more common and sounds more natural in this context.

What is the difference between ins Wasser and im Wasser?

The difference is motion vs. location, linked to accusative vs. dative with the preposition in.

ins Wasser = in das Wasser → accusative (movement into something)
- Ich werfe einen Stein ins Wasser.
  I throw a stone into the water. (the stone moves from outside to inside)
im Wasser = in dem Wasser → dative (resting in something)
- Der Stein liegt im Wasser.
  The stone is lying in the water. (no movement)

Rule of thumb with in:

Accusative → movement/change of location (into).
Dative → no movement, just location (in / inside).

What gender is Stein, and what about Wasser? How do their articles change?

Stein is masculine: der Stein.
Wasser is neuter: das Wasser.

Indefinite article (a) with Stein:

Nominative: ein Stein – A stone (is there).
Accusative: einen Stein – I see/throw a stone.

Definite article (the) with Wasser:

Nominative: das Wasser – The water is cold.
Accusative: das Wasser – Ich sehe das Wasser. (same form)
In your sentence, we have in das Wasser, which contracts to ins Wasser.

So:

einen Stein = masculine accusative object
ins Wasser = in das Wasser = into the neuter accusative object of the preposition in

Could I also say Ich werfe den Stein ins Wasser? What is the difference?

Yes, that is correct German, but the meaning changes slightly:

Ich werfe einen Stein ins Wasser.
→ I throw a stone into the water. (some unspecified stone; you haven’t identified it before)
Ich werfe den Stein ins Wasser.
→ I throw the stone into the water. (a specific stone that is already known from the context: that stone we’ve been talking about)

So:

einen Stein = indefinite, not specified
den Stein = definite, already identified

Can I change the word order, e.g. Ich werfe ins Wasser einen Stein or Einen Stein werfe ich ins Wasser?

Yes, German word order is flexible as long as the finite verb stays in second position in main clauses.

All of these are grammatically correct:

Ich werfe einen Stein ins Wasser.
(neutral, most common)
Ich werfe ins Wasser einen Stein.
(still correct; a bit unusual, but possible in spoken language for emphasis)
Einen Stein werfe ich ins Wasser.
(emphasis on a stone as opposed to something else; sounds stylistic or contrastive)

The basic rule:

The conjugated verb (werfe) must be in second position.
The elements around it can be reordered for emphasis or style.

How is the verb werfen conjugated, and is it regular or irregular?

Werfen is an irregular (strong) verb with a vowel change in some forms.

Present tense:

ich werfe – I throw
du wirfst – you throw (singular, informal)
er/sie/es wirft – he/she/it throws
wir werfen – we throw
ihr werft – you throw (plural, informal)
sie/Sie werfen – they / you (formal) throw

Notice the vowel change:

werf- → wirf- in du wirfst, er/sie/es wirft.

In your sentence, werfe is 1st person singular present: Ich werfe.

Could I use another verb like schmeißen instead of werfen?

Yes, you could say:

Ich schmeiße einen Stein ins Wasser.

Differences:

werfen – neutral, standard German; used in both spoken and written language.
schmeißen – more colloquial / informal, roughly like to chuck / to toss in English.

So:

For neutral or formal German, werfen is safer.
Among friends or in casual speech, schmeißen is very common.

Why are Stein and Wasser capitalized?

In German, all nouns are capitalized, regardless of their position in the sentence.

Stein (stone) → noun → capitalized
Wasser (water) → noun → capitalized

This is simply an orthographic rule of German: every noun starts with a capital letter. Verbs (werfen), adjectives, etc. are not capitalized unless they begin the sentence or are turned into nouns in specific constructions.

How would I make this sentence negative in German?

You have a few options, depending on what you want to negate.

You don’t throw any stone (at all) into the water:
- Ich werfe keinen Stein ins Wasser.
- Here keinen is the negative form of einen for masculine accusative.
You throw the stone, but not into the water (you negate the prepositional phrase):
- Ich werfe den Stein nicht ins Wasser.
- nicht stands before the phrase you are negating (ins Wasser).
You do not throw the stone at all (negating the action with a specific stone):
- Ich werfe den Stein nicht.

So:

Use kein- to negate an indefinite noun (einen Stein → keinen Stein).
Use nicht to negate the verb or a specific phrase like ins Wasser.

Is it possible (or natural) to say Ich werfe einen Stein in das Wasser without the contraction?

Yes, Ich werfe einen Stein in das Wasser is grammatically correct and fully understandable.

However:

In everyday language, Germans strongly prefer the contracted form ins Wasser.
in das Wasser sounds a bit more formal, careful, or sometimes contrastive (e.g. if you really want to contrast it with something like in die Luft).

So:

Normal, natural sentence: Ich werfe einen Stein ins Wasser.
Explicit / formal / contrastive context: Ich werfe einen Stein in das Wasser, nicht in die Luft.