Learn Hardcore Polish: Po bieganiu boli mnie kolano, więc robię krótszy trening. - After running my knee hurts, so I do a shorter workout.

The verb is biegać (to run, generally).
From this verb we form a verbal noun bieganie (running).
After po (meaning after), this noun must go into the locative: bieganie → bieganiu.

So:

po bieganiu = after running
You cannot say po biegać or po biegałem here. Those would be grammatically wrong in this context. The structure is:

po + [noun in locative]
po bieganiu, po pracy, po obiedzie
(after running, after work, after lunch)

What exactly is bieganiu? Is it a verb form or a noun?

Bieganiu is a noun, not a verb form.

Base verb: biegać (to run, repeatedly / as a hobby).
Verbal noun: bieganie (running).
Case: locative singular → (o) bieganiu, po bieganiu.

Polish often uses such verbal nouns where English uses -ing forms (running, swimming, walking), but grammatically in Polish they behave like regular nouns, with cases:

bieganie – nominative (running is healthy)
biegania – genitive (I’m afraid of running)
bieganiu – locative (after running / talking about running)

Why is it boli mnie kolano and not moje kolano boli?

Both are possible, but they sound slightly different:

Boli mnie kolano. – Very natural, neutral way to say “My knee hurts.”
Literally: “Hurts me (the) knee.”
Structure: verb – indirect object – subject.
Moje kolano boli. – Also correct, but puts more emphasis on “my knee”, as if you were contrasting it with something else:
- Moje kolano boli, ale kostka nie.
  (My knee hurts, but my ankle doesn’t.)

Polish often omits possessives (my/your/his) when the owner is obvious from the pronoun:

Boli mnie kolano. – it’s clear it’s my knee.
My leg hurts. → Boli mnie noga. (not normally moja noga unless emphasizing).

Why is the word order boli mnie kolano and not kolano mnie boli?

Both orders are grammatical; the difference is mostly in emphasis.

Boli mnie kolano. – neutral statement; a very standard way to say “my knee hurts.”
Kolano mnie boli. – emphasizes kolano a bit more, like:
- “It’s the knee that hurts me (not something else).”

Polish word order is relatively flexible. The core information is carried by:

boli – hurts (3rd person singular)
mnie – me
kolano – knee

So boli mnie kolano and kolano mnie boli both mean “my knee hurts”, with slight shifts in focus. In isolation, boli mnie kolano is the safest, most neutral choice.

Why is the verb boli singular? Could it ever be bolą?

Boli agrees in number with the subject, which here is kolano (knee, singular).

Boli – 3rd person singular, present
Boli mnie kolano. – My knee hurts.

If the subject is plural, you use bolą:

Bolą mnie kolana. – My knees hurt.
Boli mnie głowa. – My head hurts.
Bolą mnie plecy. – My back hurts. (literally “backs”, plural in Polish)

So:

singular body part → boli
plural body parts → bolą

What is the difference between mnie and mi? Could I say boli mi kolano?

Mnie and mi are both forms of “me”, but they differ in stress and formality:

mnie – stressed or “full” form, also more careful/neutral.
mi – clitic (weak, unstressed) form, very common in speech.

In this exact sentence:

Boli mnie kolano. – fully correct, neutral.
Boli mi kolano. – very common in everyday spoken Polish; informal but not wrong.

However, mnie is safer for learners:

It’s always correct.
It’s needed when the pronoun is emphasized or in certain positions.

For now, you can remember:

In writing and careful speech: prefer mnie.
In informal speech you’ll hear both mnie and mi.

Why is there no “ja” (I) in więc robię krótszy trening?

Polish is a pro-drop language: the subject pronoun (ja, I) is usually omitted, because person and number are already shown in the verb ending:

robię – 1st person singular → I do / I am doing.

So:

Więc robię krótszy trening. – So I do a shorter workout.
Więc ja robię krótszy trening. – also correct, but here ja is stressed, usually to contrast:
- A oni robią długi, a ja robię krótszy trening.
  (And they do a long one, and I do a shorter workout.)

Normally, just robię is enough to mean “I do”.

What does więc mean, and why is there a comma before it?

Więc means “so,” “therefore,” “thus”. It introduces a result or consequence:

Po bieganiu boli mnie kolano, więc robię krótszy trening.
After running my knee hurts, so I do a shorter workout.

The comma is there because więc connects two clauses:

Po bieganiu boli mnie kolano – first clause.
(Ja) robię krótszy trening – second clause.

In Polish, when you join two clauses with linking words like więc, ale, bo, że, you usually put a comma before them.

Why is it krótszy trening and not krótsze trening?

Adjectives in Polish must agree with the nouns in gender, number, and case.

trening – masculine, singular, accusative (as the direct object of robię).
The matching masculine accusative singular form of krótki (short) is krótszy (comparative: shorter).

So:

robię trening – I do a workout.
robię krótszy trening – I do a shorter workout.

Krótsze is a different form (neuter/masculine-personal plural etc.) and would not agree with trening here, so krótsze trening is wrong.

Why do we say trening and not ćwiczenia or something else? Does trening mean “training” in general?

Trening in modern Polish typically means a workout / training session, often physical or sports-related:

Mam trening piłki nożnej. – I have football practice.
Robię trening siłowy. – I do strength training.

Ćwiczenia is broader: exercises, not necessarily one “session”:

Robię ćwiczenia. – I’m doing exercises (could be physical, grammar, music, etc.)

In this context, krótszy trening is very natural for:

a shorter workout / training session, especially in sports / gym contexts.

You could also say:

Po bieganiu boli mnie kolano, więc robię krótsze ćwiczenia.
That would sound more like shorter exercises (a bit less like one defined workout session).

Could I say Po bieganiu bolą mnie kolana instead? What would be the difference?

Yes, that’s perfectly correct, just with a different meaning:

Po bieganiu boli mnie kolano. – After running, my knee hurts (one knee).
Po bieganiu bolą mnie kolana. – After running, my knees hurt (both / multiple).

Changes:

kolano (singular) → kolana (plural).
boli (singular verb) → bolą (plural verb).

So you choose singular or plural depending on whether one knee or both knees hurt.

Could I use a different preposition instead of po to say “after running”?

You have a couple of options, but po bieganiu is the most natural and compact:

Po bieganiu – standard, idiomatic:
- Po bieganiu boli mnie kolano.
Po tym, jak biegam / pobiegam – more explicit, with a whole clause:
- Po tym, jak biegam, boli mnie kolano.
- Po tym, jak pobiegam, boli mnie kolano.
  These sound a bit more formal or explanatory.
Kiedy (już) pobiegam, boli mnie kolano. – When I (have) run, my knee hurts.

For a simple “After running, X happens”, po + [noun in locative] is the default:

po bieganiu, po pracy, po obiedzie, po treningu.