W upalny dzień piję więcej wody.

Here upalny dzień is in the accusative singular. Although w often takes the locative when it means location, Polish also uses w + accusative in some time expressions, such as w ten dzień, w każdy piątek, and w upalny dzień.

So w upalny dzień means on a hot day.

Also, dzień is a masculine inanimate noun, so its accusative singular looks the same as its nominative singular. That is why you see dzień, not an obviously different form. The adjective matches it, so upalny also stays the same.

You may also see w upalnym dniu, but that sounds more formal or literary here.

Because after więcej (more), Polish normally uses the genitive.

So:

• więcej wody = more water
• dużo wody = a lot of water
• mało wody = little water

Here wody is genitive singular, because woda is an uncountable noun.

By contrast:

• Piję wodę = I drink water

In that sentence, wodę is the normal direct object in the accusative. But once you add a quantity word like więcej, the noun changes to the genitive.

It can mean either, depending on context.

Polish does not usually separate simple present and present continuous the way English does. The form piję can mean:

• I drink
• I am drinking

In this sentence, the natural meaning is the general one: I drink more water on a hot day.

Because the verb already shows who the subject is.

Piję ends in -ę, which tells you it means I drink. So ja is usually unnecessary.

You can add ja for emphasis or contrast:

• Ja piję więcej wody, a on mniej.
  I drink more water, and he drinks less.

But in a neutral sentence, leaving it out is normal.

Because this sentence describes a general habit or a repeated situation, not one completed action.

Pić is imperfective, and that is the normal choice for:

• habits
• repeated actions
• general truths

A perfective verb such as wypić would change the meaning. Its present-tense form actually has future meaning:

• wypiję = I will drink up / I will finish drinking

So W upalny dzień piję więcej wody is correct because it means something habitual.

It can mean hot days in general.

Polish often uses the singular in this kind of generic statement, just like English can say On a hot day...

So this sentence does not have to mean one particular day. It can mean a general pattern.

If you want to make the repeated meaning more explicit, you can also say:

• W upalne dni piję więcej wody.
  On hot days I drink more water.

Yes. Polish word order is quite flexible.

W upalny dzień piję więcej wody is natural because it puts the time phrase first: On a hot day...

But other orders are also possible, for example:

• Piję więcej wody w upalny dzień.
• Więcej wody piję w upalny dzień.

The basic meaning stays the same, but the emphasis shifts. Polish often moves words around to highlight different parts of the sentence.

Upalny means very hot, especially about weather. A good English match is hot or sweltering.

So:

• upalny dzień = a hot / sweltering day

It is more specifically a weather word than gorący, which is broader and can describe objects, drinks, food, and so on:

• gorąca kawa = hot coffee
• gorący talerz = a hot plate

For weather, upalny dzień is very natural.

Because Polish has no articles.

There is no separate word for a/an or the in Polish. Specificity is understood from context.

So w upalny dzień can mean:

• on a hot day
• on the hot day

In this sentence, the context makes the generic meaning on a hot day the natural one.