Learn Hardcore Polish: Po treningu idę bezpośrednio pod prysznic, bo jest mi bardzo gorąco. - After training I go directly to the shower because I am very hot.

Using po trening (accusative) would be wrong in this context. Po + accusative exists, but with other meanings, e.g. iść po chleb = go (in order) to get bread.

Why is it idę, not chodzę? Aren’t both “I go”?

Both translate as “I go”, but they’re different:

idę – one specific movement now / in the near future (single, concrete action)
- Po treningu idę pod prysznic. = After this training session, I go/am going to the shower.
chodzę – habitual or repeated action (I go regularly)
- Po treningu zawsze chodzę pod prysznic. = After training I always go to the shower.

In your sentence, idę is natural because it’s about what happens after this particular training (or a specific, planned situation), not a general habit.

Why can idę (present tense) refer to the future “after training”?

In Polish, present tense of imperfective verbs (like iść) is often used for planned or scheduled future actions, especially when there is a time expression:

Jutro idę do kina. = I’m going to the cinema tomorrow.
Po treningu idę pod prysznic. = I’m going straight to the shower after training.

So even though idę is grammatically “present”, with po treningu it’s naturally understood as future relative to the training.

Why is it pod prysznic and not do prysznica or pod prysznicem?

All three are grammatical, but they mean different things and are used differently:

pod prysznic (accusative) – direction / movement:
- iść pod prysznic = go to the shower (to take a shower)
  This is the normal idiom for “go to the shower” in Polish.
pod prysznicem (instrumental) – location:
- Jestem pod prysznicem. = I am in the shower / under the shower.
do prysznica (genitive) – also direction, but sounds odd here and is rarely used in this meaning. You’d typically say do łazienki (to the bathroom) instead, not do prysznica.

So in your sentence, pod prysznic is the natural choice.

Why does pod take accusative (prysznic) here?

Preposition pod can take:

accusative – when it expresses movement toward a place (direction)
instrumental – when it expresses location at a place

Examples:

Direction (accusative):
- Kładę książkę pod stół. = I’m putting the book under the table.
- Idę pod prysznic. = I’m going to the shower.
Location (instrumental):
- Książka leży pod stołem. = The book is lying under the table.
- Stoję pod prysznicem. = I’m standing under the shower.

Your sentence has movement, so pod takes the accusative: pod prysznic.

Could I say bezpośrednio idę pod prysznic instead of idę bezpośrednio pod prysznic?

Yes, bezpośrednio (directly) is flexible in placement:

Po treningu idę bezpośrednio pod prysznic.
Po treningu bezpośrednio idę pod prysznic.

Both are correct and natural.

The most common and neutral word order here is probably the one in your sentence:
Po treningu idę bezpośrednio pod prysznic.

What is the structure jest mi bardzo gorąco? Why mi and not ja?

This is a very common impersonal construction in Polish:

jest mi gorąco = literally: “it is hot to me”
→ natural English: I feel hot / I am very hot

Key points:

mi is dative (to me), a short form of mnie:
- jest mi / jest mnie – it is to me
  Used for physical/mental states and feelings:
- jest mi zimno – I’m cold
- jest mi smutno – I feel sad
- jest mi przykro – I’m sorry / I feel bad about it
Subject is implicit / impersonal – there is no clear “it”.
Polish often uses this pattern instead of “I am X” with feelings/sensations.

We do not say *jestem gorąco.
Jestem gorący would sound like “I am hot” (physically warm to the touch, or even “sexy”), not like “I feel hot”. For how you feel, the idiomatic form is jest mi gorąco.

What’s the difference between mi and mnie here? Could I say bo jest mnie bardzo gorąco?

Here you should say mi, not mnie:

mi – unstressed, clitic form; very common in the middle of a sentence:
- bo jest mi bardzo gorąco
mnie – stressed form; used when you want to emphasize me or when standing alone:
- To mnie jest gorąco, nie jemu. = It’s me who’s hot, not him.
- Komu jest gorąco? — Mnie.

So:

bo jest mi bardzo gorąco – normal, neutral
bo jest mnie bardzo gorąco – sounds wrong/unnatural in this position

Why is it gorąco and not gorący or gorąca?

In jest mi gorąco, the word gorąco is an adverb-like form (or a neuter predicate form) derived from the adjective gorący (hot).

Polish often uses such -o forms in impersonal expressions about:

weather:
- Jest ciepło. – It’s warm.
- Jest zimno. – It’s cold.
how someone feels (with dative):
- Jest mi zimno. – I’m cold.
- Jest mi gorąco. – I’m hot.

Using the adjectival forms gorący/gorąca/gorące here would sound odd or change the meaning. For I feel hot/cold, use jest mi gorąco/zimno.

What does bo do here? How is it different from ponieważ or dlatego że?

bo is a very common conjunction meaning “because”:

Idę pod prysznic, bo jest mi bardzo gorąco.

Differences:

bo – neutral, very frequent in speech and writing, slightly less formal than ponieważ, but perfectly acceptable in most contexts.
ponieważ – also “because”, feels a bit more formal or bookish:
- Idę pod prysznic, ponieważ jest mi bardzo gorąco.
dlatego że – “because / for the reason that”, often used when emphasizing the cause:
- Idę pod prysznic, dlatego że jest mi bardzo gorąco.

In everyday conversation, bo is by far the most common choice.

Why is there a comma before bo?

In standard written Polish, you normally put a comma before “bo” because it introduces a subordinate clause of reason:

Idę pod prysznic, bo jest mi bardzo gorąco.

Pattern:

[main clause] , bo [reason clause].

So the comma is required here by Polish punctuation rules.

Why is there no ja (“I”) in idę? Could I say Ja idę bezpośrednio pod prysznic?

In Polish, the personal pronoun is usually omitted because the verb form already shows the person:

idę already means “I go / I am going”
So Po treningu idę… is completely normal and the most common way to say it.

You can add ja:

Po treningu ja idę bezpośrednio pod prysznic.

…but then ja adds emphasis, like:

“As for me, I go straight to the shower (maybe others don’t).”

So:

Neutral: Po treningu idę bezpośrednio pod prysznic.
Emphatic/contrasting: Po treningu ja idę bezpośrednio pod prysznic.

What does trening mean exactly? Is it like “exercise” or “workout”?

trening is usually:

training / practice / workout – often in a sports or fitness context.

Examples:

Mam dziś trening piłki nożnej. – I have football practice today.
Idę na trening na siłownię. – I’m going to a workout at the gym.

More general exercise can also be ćwiczenia (exercises):

robić ćwiczenia – do exercises (in the gym, in a textbook, etc.)

In your sentence, po treningu is naturally understood as “after (my) workout / training session.”