Myję włosy szamponem po treningu, zanim usiądę do kolacji.

In Polish, włosy is normally used in the plural when talking about the hair on someone’s head.

So:

• Myję włosy = I wash my hair
• literally: I wash hairs

This is completely normal in Polish. The singular włos means a hair or one strand of hair.

So:

• Mam długie włosy = I have long hair
• Znalazłem włos w zupie = I found a hair in the soup

Polish often leaves out possessive words like my, your, his, etc. when it is already obvious whose body part is meant.

So Myję włosy naturally means I wash my hair, not someone else’s.

Adding moje is possible, but it usually adds emphasis or contrast:

• Myję moje włosy, nie perukę = I’m washing my hair, not the wig

In an ordinary sentence, myję włosy is the most natural choice.

Because szamponem is in the instrumental case, which is often used for the tool, means, or substance used to do something.

Here it means:

• myć szamponem = to wash with shampoo

So:

• szampon = shampoo
• szamponem = with shampoo

This is a very common pattern in Polish:

• Piszę długopisem = I write with a pen
• Kroję nożem = I cut with a knife
• Myję włosy szamponem = I wash my hair with shampoo

It is the accusative plural, because it is the direct object of myję.

The verb myć takes a direct object:

• myć włosy = to wash hair
• myć ręce = to wash hands
• myć samochód = to wash a car

In this specific noun, włosy, the accusative plural looks the same as the nominative plural. That is normal for many Polish inanimate nouns.

Here po means after, and with that meaning it takes the locative case.

So:

• trening = training / workout
• po treningu = after training / after the workout

This pattern is very common:

• po pracy = after work
• po obiedzie = after lunch
• po szkole = after school
• po treningu = after training

Even though treningu may look like a genitive form to an English-speaking learner, here it is functioning as the locative after po.

Usiądę is the 1st person singular future of the perfective verb usiąść.

Polish often uses a future form after zanim when the action is still in the future from the speaker’s point of view:

• zanim usiądę do kolacji = before I sit down to dinner

This is different from English, which usually says before I sit down, not before I will sit down. Polish does not follow the same rule here.

Also, the perfective verb usiąść fits well because it refers to a single completed action: the moment of sitting down.

Because the two verbs are describing different kinds of actions.

• myję comes from myć and is imperfective. It presents the action as a process or routine.
• usiądę comes from usiąść and is perfective. It presents the action as a single completed event.

In this sentence, the meaning is probably something like a routine:

• I wash my hair with shampoo after training, before I sit down to dinner.

That is why myję works well.

If you wanted to describe one specific future occasion, you would more likely say:

• Umyję włosy szamponem po treningu, zanim usiądę do kolacji.

Here umyję would mean I will wash my hair on that particular occasion.

Because Polish usually drops subject pronouns when the verb form already shows who the subject is.

• myję already means I wash / I am washing
• usiądę already means I will sit down

So ja is unnecessary unless you want emphasis or contrast.

For example:

• Ja myję włosy, a on robi kolację.
  I’m washing my hair, and he’s making dinner.

Without emphasis, leaving out ja sounds more natural.

Usiąść do kolacji is an idiomatic Polish expression meaning to sit down to dinner or to begin dinner by sitting down for it.

The preposition do is part of this expression. It takes the genitive case, so:

• kolacja = dinner / supper
• do kolacji = to dinner

Similar expressions are:

• usiąść do stołu = to sit down at the table
• usiąść do obiadu = to sit down to lunch/dinner
• usiąść do kolacji = to sit down to supper/dinner

So this is not just the basic physical idea of sitting. It strongly suggests starting the meal.

This is a very common question.

• myć się = to wash oneself
• myć włosy = to wash one’s hair

When you specify the body part directly, Polish usually just uses the object:

• Myję ręce = I wash my hands
• Myję twarz = I wash my face
• Myję włosy = I wash my hair

You can also hear myję sobie włosy, which is colloquial and means something like I’m washing my hair with a slight personal/benefactive nuance.

But myję się włosy is not correct.

Yes. Polish word order is fairly flexible, although the original version sounds natural and neutral:

• Myję włosy szamponem po treningu, zanim usiądę do kolacji.

You could also say:

• Po treningu myję włosy szamponem, zanim usiądę do kolacji.

This gives a bit more prominence to after training.

Or:

• Myję po treningu włosy szamponem, zanim usiądę do kolacji.

This is grammatically possible, but less natural stylistically.

So yes, the order can change, but not every version sounds equally smooth. The original is a good default sentence.

Most naturally, it sounds like a habit or a general routine:

• I wash my hair with shampoo after training, before I sit down to dinner.

That is because Polish present-tense imperfective verbs often describe repeated actions or routines.

If you wanted to stress that it is happening right now, you would usually need context, for example:

• Teraz myję włosy szamponem. = I’m washing my hair with shampoo now.

If you wanted a single future event, Polish would usually prefer:

• Umyję włosy szamponem po treningu, zanim usiądę do kolacji. = I’ll wash my hair with shampoo after training, before I sit down to dinner.