Я испачкал рубашку соусом.

Испачкать is the infinitive, meaning to stain / to make dirty.

In the sentence, you need a finite verb, not an infinitive, so Russian uses the past-tense form испачкал. This form tells you:

• the action is in the past
• the subject is singular
• the speaker is masculine

If the speaker were female, it would be испачкала.
If it were we/they, it would be испачкали.

Also, in Russian past tense, the verb does not show person, so я is what tells you the subject is I.

Because рубашку is in the accusative case, which is the case usually used for a direct object.

Here, the shirt is the thing that got stained, so it is the direct object of испачкал.

• nominative: рубашка = shirt
• accusative: рубашку = shirt as the object of the verb

So:

• рубашка = the shirt is doing something / being talked about as the subject
• рубашку = someone did something to the shirt

Because Russian often uses the instrumental case to show the means, material, or substance involved in an action.

So соусом here means something like:

• with sauce
• by means of sauce
• using sauce
• in sauce / with sauce on it, depending on context

This is a very normal Russian pattern after verbs like испачкать.

Compare:

• резать ножом = to cut with a knife
• писать карандашом = to write with a pencil
• испачкать соусом = to stain with sauce

Even though sauce is not really a tool like a knife, Russian still uses instrumental for the substance that caused the stain.

Because the instrumental case can work without a preposition to express with / by means of / using.

So соусом by itself can mean with sauce in this sentence.

This is different from с соусом, which often means with sauce in the sense of accompaniment, not the substance causing the stain.

For example:

• паста с соусом = pasta with sauce
• испачкать рубашку соусом = to stain the shirt with sauce

So in your sentence, no preposition is needed.

Испачкал is perfective, which means the action is viewed as a completed whole, with a clear result.

So Я испачкал рубашку соусом means:

• the action happened
• it is complete
• the result is that the shirt ended up stained

If you used пачкал, that would be imperfective and would sound more like:

• I was staining the shirt with sauce
• I used to stain the shirt with sauce
• I kept getting the shirt dirty with sauce

For one completed event, испачкал is the natural choice.

Yes, sometimes.

Russian often leaves out subject pronouns when the meaning is clear from context. However, in the past tense, the verb does not show person, only gender and number. So испачкал tells you masculine singular, but not specifically I.

That means:

• Я испачкал рубашку соусом = fully clear on its own
• Испачкал рубашку соусом = possible in context, but by itself it can sound incomplete or context-dependent

So yes, я can be omitted, but it is often kept here for clarity.

Because Russian has no articles.

So рубашку can mean:

• a shirt
• the shirt
• sometimes even my shirt, if context makes that obvious

And соусом can mean:

• with sauce
• with the sauce

English has to choose a/the, but Russian usually leaves that to context.

It is not stated explicitly in the sentence.

Russian often leaves possession unstated when it is obvious from context, especially with things closely associated with a person, like clothing or body parts.

So Я испачкал рубашку соусом literally means I stained a/the shirt with sauce, but in real context it may naturally be understood as my shirt.

If you want to say my shirt explicitly, you could say:

• Я испачкал свою рубашку соусом = I stained my shirt with sauce
• Я испачкал мою рубашку соусом = also possible, but свою is usually more natural when the shirt belongs to the subject

A useful rule: when the subject owns the thing, Russian often prefers свой.

Yes. Russian word order is flexible because the cases show what each word is doing.

The neutral order here is:

• Я испачкал рубашку соусом.

But you could also hear:

• Рубашку я испачкал соусом.
• Соусом я испачкал рубашку.

These all keep roughly the same basic meaning, but the emphasis changes:

• Я испачкал рубашку соусом = neutral
• Рубашку я испачкал соусом = emphasis on the shirt
• Соусом я испачкал рубашку = emphasis on the sauce

So the default order is useful to learn first, but other orders are possible.

Not necessarily.

Russian is just marking the substance that caused the stain. The sentence itself does not tell you whether it was accidental or deliberate.

So Я испачкал рубашку соусом can simply mean that sauce got on the shirt and made it dirty.

If you wanted to make the accident clearer, Russian might use other wording too, but your sentence by itself does not imply intention.

Yes, very often.

Both испачкал and запачкал can mean stained / got dirty / dirtied. In everyday speech, запачкать is also very common.

For many situations, the difference is small enough that both work:

• Я испачкал рубашку соусом.
• Я запачкал рубашку соусом.

Both sound natural. A learner should mainly understand that both verbs can express the idea of getting something dirty, though exact nuance may depend on context and speaker preference.