Я замочил рубашку в воде, потому что на рубашке было пятно.

Here замочил means soaked or put in water to soak.

It comes from the verb pair:

• замачивать — imperfective
• замочить — perfective

So Я замочил рубашку means the action is seen as completed: I soaked the shirt / I put the shirt in water to soak.

This is a little more specific than just made wet. It suggests deliberately putting something in water, often to remove dirt or stains.

Because рубашку is in the accusative case.

The noun рубашка is the direct object of the verb замочил — it is the thing that was soaked.

• nominative: рубашка
• accusative: рубашку

For feminine nouns ending in -а, the accusative singular usually changes -а to -у.

So:

• рубашка = shirt as a subject
• замочил рубашку = soaked the shirt as an object

Because after в meaning in, Russian uses the prepositional case for location.

• вода = water
• prepositional singular: в воде = in water

So замочил рубашку в воде literally means soaked the shirt in water.

This phrase tells you where / in what medium the shirt was soaked.

Because a stain is understood as being on the surface of the shirt, not inside it.

So Russian uses:

• на рубашке = on the shirt

This is very natural with things like stains, spots, dirt, writing, buttons, and so on.

Examples:

• на рубашке пятно = there is a stain on the shirt
• на столе книга = there is a book on the table

Using в рубашке would mean in the shirt, which does not fit this meaning.

Because пятно is a neuter noun.

Russian past tense agrees with gender in the singular:

• был — masculine
• была — feminine
• было — neuter

Since пятно is neuter, you say:

• было пятно = there was a stain

So the verb form matches the gender of пятно.

It is in the nominative singular.

In на рубашке было пятно, the phrase works like there was a stain on the shirt. The noun пятно is the thing that existed or was present, so it stays in the nominative.

• пятно = nominative singular
• plural would be пятна

This kind of structure is very common in Russian:

• на столе была книга = there was a book on the table
• в комнате было окно = there was a window in the room

Yes, you could use a pronoun.

The sentence repeats рубашка for clarity:

• Я замочил рубашку в воде, потому что на рубашке было пятно.

But a more natural shorter version in conversation could be:

• Я замочил рубашку в воде, потому что на ней было пятно.

Here:

• на ней = on it / on her
• it refers back to рубашка

Both are correct. Repeating the noun is clear and explicit; using на ней sounds less repetitive.

Because потому что introduces a subordinate clause meaning because.

Russian normally puts a comma before it:

• Я замочил рубашку в воде, потому что на рубашке было пятно.

This is standard punctuation.

So the sentence has:

• main clause: Я замочил рубашку в воде
• subordinate clause: потому что на рубашке было пятно

Yes. Russian word order is more flexible than English, although not completely free.

The original sentence is neutral and clear:

• Я замочил рубашку в воде, потому что на рубашке было пятно.

You could also say:

• Потому что на рубашке было пятно, я замочил её в воде.

This puts more emphasis on the reason first.

Russian word order often changes the focus or emphasis, not the basic grammatical meaning.

Because Russian does not have articles like English a and the.

So:

• рубашка can mean a shirt or the shirt
• пятно can mean a stain or the stain

The exact meaning depends on context.

In this sentence, English might naturally say:

• I soaked the shirt in water because there was a stain on the shirt or
• I soaked the shirt in water because the shirt had a stain

Russian does not need separate article words to express that.

Yes, but the meaning changes.

• замочил — perfective: completed action
• замачивал — imperfective: process, repetition, or background action

Compare:

• Я замочил рубашку в воде = I soaked the shirt / I put the shirt in water to soak
  → one completed action

• Я замачивал рубашку в воде = I was soaking the shirt in water / I used to soak the shirt in water
  → focuses on the process or repeated action

In your sentence, замочил is the natural choice because it describes a single completed action done for a reason.

No, not exactly.

• замочить рубашку = to soak the shirt
• стирать рубашку = to wash the shirt

Soaking is often just one step before washing, especially if there is a stain.

That is why this sentence says:

• Я замочил рубашку в воде, потому что на рубашке было пятно.

It suggests: I soaked the shirt because there was a stain on it, probably to help remove the stain.