Learn Hardcore Polish: Po obiedzie boli mnie brzuch, bo zjadłem za dużo ciastek przy ognisku. - After lunch my stomach hurts, because I ate too many cookies by the campfire.

You do sometimes use po + accusative with certain verbs (e.g. iść po chleb – to go for bread), but in the meaning “after (something in time)”, po takes the locative.

Why is it boli mnie brzuch and not something like mój brzuch boli? What is the word order here?

Boli mnie brzuch is the most natural, neutral way to say “my stomach hurts” in Polish.

Literally it is:

boli – hurts (3rd person singular, from boleć)
mnie – me (object pronoun)
brzuch – stomach (subject, nominative)

Literal structure: “hurts me stomach” → “my stomach hurts”

You can say:

Boli mnie mój brzuch. – grammatically OK but sounds redundant (we already know it’s “my” from mnie).
Brzuch mnie boli. – also very common and natural; the emphasis is slightly different (more focus on brzuch).

So typical patterns are:

Boli mnie brzuch.
Brzuch mnie boli.

You normally don’t need mój here.

Is mnie here the same as mi? Could I say Boli mi brzuch?

Both mnie and mi are forms of “me”, but they differ in emphasis and style:

mnie – full form, used for emphasis or in neutral/standard style.
mi – clitic (short) form, often less stressed; very common in speech.

With boleć, Poles commonly say:

Boli mnie brzuch. – neutral, standard.
Brzuch mnie boli. – also neutral.
Brzuch mi boli. – heard in colloquial speech, but many consider “Boli mnie brzuch” stylistically better / more correct.

Boli mi brzuch is widely used in informal speech, but if you’re learning, it’s safer (and always correct) to stick to:

Boli mnie brzuch.

Why is it brzuch and not brzucha or some other form?

In Boli mnie brzuch, brzuch is in the nominative case, because it is the subject of the verb boleć:

brzuch (nominative singular, subject) boli (hurts).

Compare:

Brzuch boli. – The stomach hurts.
Boli mnie brzuch. – My stomach hurts (literally “hurts me stomach”).

Forms would be:

Nominative: brzuch – used here.
Genitive: brzucha – e.g. Nie mam brzucha. – I don’t have a belly.
Accusative: brzuch (same as nominative for masculine inanimate).

So we use brzuch because it’s the subject in nominative.

What is the difference between bo and ponieważ? Could I replace bo with ponieważ here?

Yes, you can say:

Po obiedzie boli mnie brzuch, ponieważ zjadłem za dużo ciastek przy ognisku.

bo and ponieważ both mean “because”, but:

bo – shorter, more colloquial, very common in everyday speech and writing.
ponieważ – a bit more formal / bookish; still very common and perfectly correct.

In this sentence both are fine. bo sounds more natural in casual conversation.

Why is there a comma before bo?

In Polish, you normally put a comma before bo when it introduces a full subordinate clause (a reason):

… , bo zjadłem za dużo ciastek przy ognisku.

This is a standard punctuation rule: when bo means “because” and starts a new clause, use a comma before it.

You would not separate with a comma if bo were part of another structure (rare in modern Polish), but in sentences like this, you almost always need the comma.

What does zjadłem tell us about the subject? Why not jadłem?

zjadłem is:

1st person singular, masculine, past tense.
from zjeść – perfective verb “to eat (up), to have eaten”.

So:

zjadłem – I (male) ate / I have eaten (the action is completed).
zjadłam – I (female) ate.

jadłem/jadłam are from jeść (imperfective) and focus on the process:

Jadłem ciastka. – I was eating cookies / I used to eat cookies.
Zjadłem ciastka. – I ate the cookies (I finished them).

Here the point is that the eating is completed and caused stomach pain, so zjadłem is the natural choice.

How do I know I should use za dużo with the genitive (ciastek)?

In Polish, expressions of quantity like:

dużo – a lot of
mało – little/few
za dużo – too much/too many
za mało – too little/few

are usually followed by the genitive case.

So:

dużo ciastek – a lot of cookies
za dużo ciastek – too many cookies

The noun ciastko (cookie) in plural genitive becomes ciastek:

Nominative plural: ciastka
Genitive plural: ciastek

Because of za dużo, you need ciastek (genitive), not ciastka.

What’s the difference between ciastka and ciastek?

They are different cases and numbers of the noun ciastko (cookie):

ciastko – one cookie (nominative singular).
ciastka – cookies (nominative plural) or cookies (accusative plural).
ciastek – cookies (genitive plural).

Examples:

Lubię ciastka. – I like cookies. (accusative plural)
Nie mam ciastek. – I don’t have any cookies. (genitive plural)
Zjadłem za dużo ciastek. – I ate too many cookies. (genitive plural after za dużo)

So ciastka = “cookies” as the subject or direct object;
ciastek = “of cookies” after quantity words or in negative possession, etc.

Why is it przy ognisku and not na ognisku? What does przy mean here?

przy means “by / next to / near” and takes the locative case:

przy ognisku – by the campfire / near the campfire.

ognisko (campfire) → locative singular: ognisku.

na ognisku would literally be “on the fire” (physically on top of it), which is usually not what you want.

So:

siedzieliśmy przy ognisku – we were sitting by the campfire.
use przy for “next to, at (a place)” in this sense.

Why ognisku and not ognisko after przy?

przy requires the locative case.

Nominative: ognisko
Locative: ognisku

So:

przy ognisku – by the campfire.
w ognisku – in the fire (also locative: ognisku).
o ognisku – about the campfire (again locative).

The change -o → -u is a regular pattern for many neuter nouns in the locative singular.

Can I change the word order, for example: Po obiedzie brzuch mnie boli, bo za dużo ciastek zjadłem przy ognisku?

Yes, Polish word order is quite flexible, and your version is grammatically correct:

Po obiedzie brzuch mnie boli, bo za dużo ciastek zjadłem przy ognisku.

Possible variants include:

Po obiedzie boli mnie brzuch, bo zjadłem za dużo ciastek przy ognisku. (original)
Po obiedzie brzuch mnie boli, bo zjadłem za dużo ciastek przy ognisku.
Brzuch mnie boli po obiedzie, bo zjadłem za dużo ciastek przy ognisku.

The basic information stays the same, but you shift emphasis slightly:

Starting with Po obiedzie highlights when it happens.
Starting with Brzuch highlights what hurts.
Moving zjadłem earlier (… bo zjadłem za dużo ciastek …) is the most neutral; putting it last (… bo za dużo ciastek zjadłem …) gives it a bit of emphasis, like “…because I really did eat too many cookies.”

All of these are natural-sounding for a native speaker.