W upalne popołudnie lubię siedzieć w cieniu drzewa z książką i zimną wodą.

Because here w is being used in a time expression, not a location expression.

In Polish, w + accusative can mean on / during with certain time phrases:

• w tę noc
• w chłodny wieczór
• w sobotę

So w upalne popołudnie means on a hot afternoon / during a hot afternoon.

If this were about physical location, w would normally take the locative:

• w domu
• w mieście
• w cieniu

So the case changes because the function changes:
time -> often accusative here
location -> usually locative

Because upalne agrees with popołudnie, and popołudnie is a neuter singular noun.

Polish adjectives must match the noun in:

• gender
• number
• case

Here:

• popołudnie = neuter singular
• in this sentence it is in the accusative
• for neuter singular, the adjective ending is -e

So:

• upalne popołudnie
• zimne mleko
• małe dziecko

This same ending often appears in both the nominative and accusative singular for neuter nouns.

Popołudnie is neuter.

A useful rule of thumb is that many Polish nouns ending in -o, -e, or -ę are neuter, and popołudnie fits that pattern.

You can see its neuter gender in adjective agreement:

• upalne popołudnie
• not upalny popołudnie
• not upalna popołudnie

Knowing the gender helps you choose the right adjective endings and verb forms in other sentences.

Because after lubić Polish usually uses the infinitive when you say you like doing an action.

So:

• Lubię siedzieć = I like sitting / I like to sit
• Lubię czytać = I like reading / I like to read
• Lubię pływać = I like swimming / I like to swim

This is very normal Polish structure:

• lubić + infinitive

Compare:

• Lubię książki = I like books
• Lubię czytać książki = I like reading books

Because siedzieć describes the state of being seated, while usiąść means to sit down once, as a completed action.

So:

• siedzieć = to be sitting
• usiąść = to sit down

In this sentence the speaker likes the ongoing activity or situation, not the single moment of taking a seat. That is why siedzieć sounds right.

Also, after lubię, Polish often prefers an imperfective verb when talking about general habits or repeated enjoyment:

• Lubię siedzieć w cieniu
• Lubię czytać
• Lubię spacerować

Because w cieniu expresses location: being in the shade.

With w, Polish often distinguishes:

• location -> locative
• direction / movement into -> accusative

Here the verb is siedzieć, which is static, not directional. So the locative is used:

• w cieniu = in the shade

Compare:

• siedzieć w cieniu = to sit in the shade
• wejść w cień = to go into the shade

The noun cień changes in the locative:

• nominative: cień
• locative: cieniu

Because cień often takes a noun in the genitive to show what is casting the shade.

So:

• cień drzewa = the shade/shadow of a tree
• cień domu = the shade/shadow of a house
• cień góry = the shadow of a mountain

Here:

• basic noun: drzewo = tree
• genitive singular: drzewa

So w cieniu drzewa literally means in the shade of a tree.

Because after z meaning with, Polish uses the instrumental case.

So:

• z książką = with a book
• z wodą = with water

Forms here:

• książka -> książką
• woda -> wodą

This is a very common pattern:

• z przyjacielem = with a friend
• z herbatą = with tea
• z rodziną = with family

Be careful: z does not always take the same case.
For example:

• z książką = with a book -> instrumental
• z domu = from the house -> genitive

Because the adjective must agree with wodą, which is in the instrumental singular.

So:

• woda = nominative
• wodą = instrumental

The adjective changes too:

• zimna woda = cold water
• z zimną wodą = with cold water

That is standard adjective agreement in Polish:

• z dobrą kawą
• z ciekawą książką
• z zimną wodą

Usually it means something broad like with a book and some cold water, meaning they are part of the situation: in your hands, beside you, or available to you.

It does not necessarily mean you are physically holding both the whole time. It is more like English:

• sitting in the shade with a book and cold water

Also, zimna woda can naturally mean cold water as a drink, even if Polish does not explicitly say a glass of or a bottle of water here.

Yes. Polish word order is fairly flexible, and changing it usually changes emphasis, not the core meaning.

The given sentence starts with W upalne popołudnie, which sets the scene first:

• W upalne popołudnie lubię siedzieć... This feels natural and slightly literary or descriptive.

You could also say:

• Lubię siedzieć w cieniu drzewa z książką i zimną wodą w upalne popołudnie.

That is still correct, but the rhythm and emphasis are different. The original version foregrounds the time expression.

Because Polish has no articles.

So Polish does not normally have separate words corresponding to English a/an and the.

That means:

• drzewa could mean of a tree or of the tree
• książką could mean with a book or with the book
• zimną wodą could mean with cold water or with the cold water

Context tells you which is meant. This is one of the biggest differences from English.

Not exactly.

• w cieniu drzewa = in the shade of a tree
• pod drzewem = under a tree

These often overlap, but they are not identical.

For example:

• You can be pod drzewem even if the sun is not being blocked much.
• You can be w cieniu drzewa slightly away from the trunk, as long as you are in the tree’s shade.

So w cieniu drzewa focuses on the shade, while pod drzewem focuses on the physical position under the tree.