Po pracy boli mnie oko.

Because the preposition po takes the locative case when it means after.

• praca = work
• locative singular of praca = pracy

So:

• po pracy = after work

This is a very common pattern in Polish:

• po szkole = after school
• po obiedzie = after dinner
• po lekcji = after the lesson

Here it means after work or when work is finished.

Since Polish has no articles, po pracy can sometimes mean either after work or after the work, depending on context. In an everyday sentence like this, the most natural interpretation is simply after work.

Polish uses a different pattern from English with boleć.

A very literal breakdown is:

• po pracy = after work
• boli = hurts
• mnie = me
• oko = eye

So the structure is basically After work, the eye hurts me.

In natural English, we say My eye hurts, but in Polish it is very normal to say it with:

• the body part as the grammatical subject
• the person affected as mnie, cię, go, etc.

This is why boli mnie oko sounds natural.

Here mnie is functioning as the person affected by the pain, and in this construction with boleć, Polish normally uses the accusative form.

So:

• boli mnie głowa = my head hurts
• bolą mnie plecy = my back hurts / my back aches
• boli mnie oko = my eye hurts

Mi is the short dative form, so it is not the standard choice in this pattern.

Oko is in the nominative singular.

It is the grammatical subject of boli, because it is the thing that is doing the hurting in Polish grammar.

One thing that can confuse learners is that for neuter nouns like oko, the nominative and accusative singular look the same. But here it is nominative because it is the subject.

Because oko is singular.

The verb agrees with the thing that hurts:

• oko = eye → singular → boli
• oczy = eyes → plural → bolą

So:

• Boli mnie oko. = My eye hurts.
• Bolą mnie oczy. = My eyes hurt.

In Polish, with body parts, the possessive word is often left out if it is obvious who the body part belongs to.

So:

• Boli mnie oko naturally means My eye hurts
• not just An eye hurts me

The pronoun mnie already shows who is affected, so Polish does not usually need moje.

Using moje is possible, but it adds emphasis or contrast:

• Moje oko boli bardziej niż lewe. = My eye hurts more than the left one.

Yes, it is understandable and grammatical, but it sounds less natural in many everyday situations.

The most idiomatic everyday Polish is:

• Boli mnie oko.

Moje oko boli puts more focus on my eye itself, almost as if contrasting it with something else. So it is possible, but not the most typical neutral way to say it.

No, Polish word order is quite flexible.

These are all possible, with slightly different emphasis:

• Po pracy boli mnie oko. = neutral, with after work as the setting
• Boli mnie oko po pracy. = also natural
• Oko mnie boli po pracy. = more emphasis on eye

The version you have, Po pracy boli mnie oko, is a very natural way to begin with the time expression and then say what happens.