Learn Hardcore Polish: Po burzy fale na morzu są wysokie, a niebo nad dachem domu jest jeszcze ciemne. - After the storm the waves on the sea are high, and the sky above the roof of the house is still dark.

You also use po + locative for many location expressions (e.g. po ulicy – along the street), but here it’s clearly temporal: “after the storm.”

Why is fale used (plural) instead of a singular word for “wave”?

Polish usually uses the plural fale (waves) for talking about the sea, because we naturally imagine many waves, not one isolated wave.

fala – one wave
fale – waves

So:

Fale na morzu są wysokie = The waves on the sea are high

Using a singular subject like fala na morzu jest wysoka would feel like you are describing one specific wave, which is not what is meant here.

Why is it na morzu and not w morzu?

Polish normally uses na with large open surfaces or open bodies of water:

na morzu – on the sea / at sea
na jeziorze – on the lake
na oceanie – on the ocean

w is more “inside something”:

w wodzie – in the water
w basenie – in the swimming pool

So fale na morzu is the standard, idiomatic phrase for waves on the sea.

Why is it na morzu (with -u) and not na morze?

Again, na here expresses a location (“on the sea”), not movement. For static location with na, Polish uses the locative case:

Nominative: morze (sea)
Locative: morzu

Compare:

Jesteśmy na morzu. – We are at sea. (locative: morzu)
Jedziemy na morze. – We are going to the sea. (accusative: morze)

In your sentence, the waves are on the sea, so we need na morzu.

Why do we say fale … są wysokie, and not something like są wysokimi?

In sentences like “X is Y (an adjective)”, the adjective in Polish stands in the nominative case, agreeing with the subject:

fale (nom. pl.)
wysokie (nom. pl. non-masculine-personal)

So:

Fale są wysokie. – The waves are high.

Instrumental (wysokimi) would be used with nouns in “X is Y (a noun)” structures, not in simple adjective predicates:

On jest nauczycielem. – He is a teacher. (nauczycielem – instrumental)
On jest wysoki. – He is tall. (wysoki – nominative adjective)

Why is the adjective wysokie and not wysokie with some other ending (e.g. wysokie vs wysokie)? What does it agree with?

Wysokie agrees with fale:

fale – feminine plural
For feminine plural (and generally all non‑masculine‑personal plural nouns) the adjective form is -e: wysokie.

Examples:

wysokie fale – high waves
małe dzieci – small children
ciemne okna – dark windows

So fale są wysokie is “waves are high,” with full gender/number agreement.

Why do we use a between the clauses, and not i?

Both a and i can be translated as and, but they differ slightly:

i – neutral “and”, simply adds information.
a – often marks a contrast or a shift to a different but related scene/subject.

Here:

Po burzy fale na morzu są wysokie, a niebo nad dachem domu jest jeszcze ciemne.

The a suggests a contrast or at least a separate “scene”: the state of the sea vs the state of the sky above the house. You could use i, but a sounds more natural and descriptive here, highlighting the two different images.

Could the word order be Po burzy są wysokie fale na morzu? Does that change the meaning?

Yes, you can change the word order; Polish is flexible. Some examples:

Po burzy fale na morzu są wysokie. (original, very natural)
Po burzy na morzu fale są wysokie.
Po burzy są wysokie fale na morzu.

All basically mean the same thing: After the storm, the waves on the sea are high.

Changing word order can lightly shift the emphasis (what you want to stress), but it doesn’t create a big meaning change here. The original order simply sounds neutral and clear.

Why is it niebo nad dachem domu, and not nad dach domu or nad dachu domu?

There are two structures here:

nad dachem – “over/above the roof”
- The preposition nad with a static location takes the instrumental case.
- Nominative: dach (roof)
- Instrumental: dachem
dach domu – “the roof of the house”
- domu is genitive singular (of the house).

So:

nad dachem domu = over the roof of the house
nad dach domu / nad dachu domu – incorrect case forms here.

Why is domu in the genitive case?

Domu is the genitive singular of dom. In dach domu:

dach = roof (nominative)
domu = of the house (genitive)

Polish commonly uses the genitive to express possession or a whole-part relationship:

dach domu – the roof of the house
drzwi samochodu – the door of the car
okno pokoju – the window of the room

So nad dachem domu literally: above the roof of the house.

Why is it niebo … jest ciemne, not ciemny or ciemna?

The noun niebo is neuter singular:

Nominative: niebo – sky

Adjectives must match the noun in gender, number, and case. For neuter singular, the nominative adjective ending is usually -e:

ciemne niebo – dark sky
niebo jest ciemne – the sky is dark

So:

ciemny – masculine singular (e.g. ciemny las – dark forest)
ciemna – feminine singular (e.g. ciemna noc – dark night)
ciemne – neuter (and some plural) (e.g. ciemne niebo)

What is the role of jeszcze in jest jeszcze ciemne? Where can it go?

Jeszcze means “still” here: is still dark.

Typical position is before the adjective or just after the verb:

niebo jest jeszcze ciemne – very natural
niebo jeszcze jest ciemne – also possible, with slightly more emphasis on jeszcze

It modifies the whole state “being dark,” telling us the sky continues to be dark, even though maybe we expect it to be getting lighter after the storm.

Could we say Niebo jest jeszcze ciemno instead of ciemne?

No; ciemno is an adverb, used in impersonal expressions like:

Jest ciemno. – It is dark. (no specific subject)

But when you have a concrete noun like niebo, you need an adjective that agrees with it:

Niebo jest ciemne. – The sky is dark. (correct)
Niebo jest ciemno. – incorrect

Similarly:

W pokoju jest jasno. – It is bright in the room. (impersonal)
Pokój jest jasny. – The room is bright. (with noun subject)

Why do we have są in the first part and jest in the second part?

They agree with the grammatical number of their subjects:

fale – plural → są (3rd person plural of być – to be)
- Fale … są wysokie. – The waves are high.
niebo – singular → jest (3rd person singular of być)
- Niebo … jest jeszcze ciemne. – The sky is still dark.

Polish verbs must agree in number (singular/plural) with the subject.

Can this whole sentence be used both for “now” and for a general description?

Yes. Without a specific time marker, the present tense in Polish can describe:

A current situation:
- You’re looking at the sea right after a storm:
  Po burzy fale na morzu są wysokie, a niebo nad dachem domu jest jeszcze ciemne.
A general, typical situation (like in literature or nature description):
- Describing what usually happens after a storm.

If you wanted a specific past event, you’d most likely use the past tense:

Po burzy fale na morzu były wysokie, a niebo nad dachem domu było jeszcze ciemne. – After the storm, the waves on the sea were high and the sky over the roof of the house was still dark.