Я бросаю камень в воду.

Камень is in the accusative case as the direct object of the verb бросаю.

• Камень is a masculine noun ending in a consonant, with a soft sign in spelling.
• For inanimate masculine nouns, the accusative singular form is identical to the nominative singular.

So the forms are:

• Nominative (subject): камень – Камень тяжёлый. (The stone is heavy.)
• Accusative (direct object): камень – Я бросаю камень. (I throw a stone.)
• Genitive: камня – У меня нет камня. (I don’t have a stone.)

You only see камня when you need genitive, not when it is a direct object of the verb бросать in a normal affirmative sentence.

The preposition в can use two different cases, and the meaning changes:

• В + accusative (в воду) = motion into / direction
  
  • Я бросаю камень в воду. – I am throwing the stone into the water.
• В + prepositional (в воде) = location (where something is)
  
  • Камень в воде. – The stone is in the water.

So:

• If something moves into the water: в воду (accusative).
• If something is already in the water, not moving: в воде (prepositional).

Бросаю is:

• Present tense
• Imperfective aspect
• 1st person singular: я бросаю

It can mean:

• I am throwing (right now)
• I throw (habitually, generally)

Compare with:

• Я брошу камень в воду.
  • Perfective aspect, future tense
  • Means I will throw a stone into the water (once, as a single completed action).
• Я бросил камень в воду.
  • Past tense, perfective
  • Means I threw a stone into the water (and it’s done).

So:

• бросать (imperfective): я бросаю, я бросал…
• бросить (perfective): я брошу, я бросил…

Yes, Russian word order is flexible.

All of these are grammatically correct:

• Я бросаю камень в воду. (neutral, very natural)
• Я бросаю в воду камень.
• Камень я бросаю в воду. (emphasis on камень)
• В воду я бросаю камень. (emphasis on в воду / the destination)

The basic information (who does what to what) is carried mainly by endings and prepositions, not by word order. Changing the order usually changes:

• Emphasis
• Style (more neutral vs more expressive/literary)

For a beginner, Я бросаю камень в воду is the best default.

You can drop я:

• Бросаю камень в воду.

Russian verbs show person and number in their endings, so бросаю already tells you it is I (1st person singular).

However:

• In neutral written language, it is more typical to keep я.
• In informal speech, diaries, quick messages, or when it’s obvious who is speaking, people often drop я:
  Сижу, думаю, бросаю камень в воду.

So both are possible; keeping я is safer and more standard.

Using the usual aspect pair бросать / бросить:

Present (imperfective):

• Я бросаю камень в воду. – I am throwing / I throw a stone into the water.

Past:

• Imperfective (process, repeated, background):
  • Я бросал камень в воду. – I was throwing / used to throw a stone into the water.
• Perfective (single completed action):
  • Я бросил камень в воду. – I threw a stone into the water.

Future:

• Compound future with imperfective (process, repeated):
  • Я буду бросать камень в воду. – I will be throwing / will throw (repeatedly).
• Simple future with perfective (single completed action):
  • Я брошу камень в воду. – I will throw a stone into the water (once).

Russian has no articles like a / an / the.

• Камень can mean a stone, the stone, or just stone depending on context.
• Я бросаю камень в воду. can be understood as:
  • I am throwing a stone into the water.
  • I am throwing the stone into the water.

If you really need to specify which stone, you add extra words:

• Я бросаю этот камень в воду. – I am throwing this stone into the water.
• Я бросаю тот камень в воду. – I am throwing that stone into the water.

Stresses:

• Я броса́ю ка́мень в во́ду.

Rough pronunciation guide (Latin letters):

• Я – ya
• бросаю – brɑ-SA-yu (stress on са)
• камень – KAH-myen’ (soft нь, like ny in canyon)
• в – short v
• воду – VO-du (stress on во; final у = oo in boot)

So the whole sentence: ya brɑ-SA-yu KAH-myen’ v VO-du.

Воду is the accusative singular of вода.

Main singular forms:

• Nominative: вода – Вода холодная. (The water is cold.)
• Accusative: воду – Я пью воду. (I drink water.) / Я бросаю камень в воду.
• Genitive: воды – Стакан воды. (A glass of water.)

Here, в воду expresses movement into the water, so в requires the accusative. That is why we get воду, not воды.

Both use the accusative to show direction, but the spatial idea is different:

• в воду – into the water, inside the volume of water
  • Я бросаю камень в воду. – The stone goes into the water.
• на воду – onto the surface of the water
  • Я бросаю камень на воду. – The stone falls onto the surface (for example, to make ripples), focus on landing on top.

So:

• в = into / in
• на = onto / on (surface)

Бросать / бросить can mean both to throw and to give up / quit, depending on context.

Literal, physical:

• Я бросаю камень в воду. – I am throwing a stone into the water.
• Он бросил мяч. – He threw the ball.

Figurative, to quit / give up:

• Я бросил курить. – I quit smoking.
• Она бросила работу. – She quit her job.
• Он бросил учёбу. – He dropped out of his studies.

In your sentence, because there is a physical object камень and a physical target в воду, the meaning is clearly the literal to throw.