Learn Hardcore Polish: Latem lubię siedzieć nad morzem i patrzeć, jak fale uderzają o piasek. - In summer I like sitting by the sea and watching how the waves hit the sand.

nad morzem – by/at the seaside, literally “by the sea”, i.e. somewhere on land next to the sea (in a coastal town, on the beach, etc.).
na morzu – at sea, out on the water (on a boat/ship).
w morzu – in the sea (physically in the water).
przy morzu – technically possible, but sounds unusual here; nad morzem is the standard idiom for “at the seaside”.

So siedzieć nad morzem = to be sitting somewhere on the shore, close to the sea but not in it or on it.

What case is morzem in nad morzem, and why that case?

Morzem is instrumental singular of morze.

The preposition nad works like this:

nad
- instrumental → location (where?):
  - Siedzę nad morzem. – I am sitting by the sea.
nad
- accusative → direction (where to?):
  - Jadę nad morze. – I am going to the seaside.

In your sentence it describes a place where you are sitting, so nad + instrumental is used: nad morzem.

Why is it lubię siedzieć and lubię … patrzeć (infinitive), not lubię siedzę or lubię patrzę?

In Polish, after verbs of liking/wanting etc., you usually use the infinitive, not another finite verb:

Lubię czytać. – I like to read / I like reading.
Chcę spać. – I want to sleep.
Muszę pracować. – I have to work.

So:

lubię siedzieć – I like to sit / I like sitting
lubię patrzeć – I like to watch

Saying *lubię siedzę or *lubię patrzę is ungrammatical. The liking verb (lubię) is conjugated; the actions you like (siedzieć, patrzeć) stay in the infinitive.

Is there any difference in meaning between siedzieć and siedzę here?

Yes:

siedzieć – infinitive: to sit, to be sitting. Used after verbs like lubić, chcieć, musieć.
siedzę – 1st person singular present: I am sitting / I sit.

Compare:

Latem siedzę nad morzem. – In summer I (actually) sit / I am sitting by the sea.
Latem lubię siedzieć nad morzem. – In summer I like sitting by the sea.

Your sentence is about preference, not a specific action right now, so the pattern lubię + infinitive is required.

Why is there a comma before jak in patrzeć, jak fale uderzają o piasek? In English we don’t always put a comma before “how/when”.

In Polish, jak here introduces a subordinate clause:

patrzeć, jak fale uderzają o piasek
= “to watch (how/when) the waves hit the sand”.

Polish punctuation rules: a clause introduced by conjunctions like że, żeby, gdy, kiedy, jak is usually separated by a comma from the main verb:

Widzę, że pada. – I see that it’s raining.
Lubię, gdy jest ciepło. – I like it when it’s warm.
Patrzę, jak fale uderzają o piasek.

So the comma is obligatory here.

Note: jak used just for comparison (e.g. taki jak, “such as/like”) often does not take a comma, but that’s a different use.

What exactly does jak mean here? Is it “how”, “when”, or “that”?

All of these are reasonable translations; jak is flexible. In this sentence it has a loose, descriptive meaning roughly like:

“as” / “when” / “how” in English.

So:

…patrzeć, jak fale uderzają o piasek
≈ “to watch as the waves hit the sand”
≈ “to watch when the waves hit the sand”
≈ “to watch how the waves hit the sand”

The exact English word you choose is about style, not different Polish meanings here.

Why is it fale uderzają, not something like fale uderzą or fale uderzyły?

Uderzają is:

tense: present
aspect: imperfective (from uderzać)
person/number: 3rd person plural

Imperfective aspect here expresses a repeated / ongoing process – the waves keep on hitting the sand. This matches the idea of a general activity you enjoy watching.

If you used the perfective uderzyć:

fale uderzą o piasek – the waves will hit the sand (once or as a single event).
fale uderzyły o piasek – the waves hit the sand (one completed event in the past).

That would not fit well with the idea of a continuous, relaxing scene you like in general.

What case is piasek after uderzają o piasek, and why is the preposition o used here?

Here:

o is a preposition meaning “against” (in the sense of physical impact).
It takes the accusative case in this meaning.
piasek (sand) is a masculine inanimate noun; its accusative = nominative (piasek).

So: uderzać o coś = to hit (against) something:

Fale uderzają o piasek. – The waves hit (against) the sand.
Samochód uderzył o drzewo. – A car hit (against) a tree.

Be careful: o + locative is used in a different meaning, e.g.:

Myślę o tobie. – I’m thinking about you.

So o can take accusative (against something) or locative (about something), depending on the verb and meaning.

Could it be uderzają w piasek instead of o piasek? Is there a difference?

Yes, uderzać w coś is also possible and grammatical, but there is a nuance:

uderzać o coś – emphasizes impact against a surface. Very natural for waves hitting the shore, a ball bouncing off a wall, etc.
uderzać w coś – emphasizes moving into/into contact with a target (often with some force), like ramming into something.

For waves and a shoreline, uderzać o piasek sounds especially idiomatic; uderzać w piasek could work, but it’s less typical and suggests more of “striking into the sand” as a solid object.

Why is it fale and not something like falami or fal?

Fale here is nominative plural of fala (a wave):

fala – 1 wave (nominative singular)
fale – waves (nominative plural)

It is the subject of the verb uderzają:

(Co?) Fale (co robią?) uderzają o piasek.
(What?) The waves (what are they doing?) hit the sand.

The other forms are different cases:

fal – genitive plural (e.g. nie ma fal – there are no waves).
falami – instrumental plural (e.g. bawić się falami – play with the waves).

In your sentence we need the subject, so we use nominative plural: fale.

Is there a difference between patrzeć na fale and patrzeć, jak fale uderzają o piasek?

Yes, they focus on slightly different things:

patrzeć na fale – literally to look at the waves. Focus: object you’re looking at.
patrzeć, jak fale uderzają o piasek – to watch as the waves hit the sand. Focus: the action / process that you see happening.

Both are correct and natural:

Latem lubię siedzieć nad morzem i patrzeć na fale.
Latem lubię siedzieć nad morzem i patrzeć, jak fale uderzają o piasek.

The original version paints a more vivid, dynamic picture.

Could I say Latem lubię siedzieć na plaży instead of nad morzem? Would that change the meaning?

Yes, it’s perfectly correct, but there is a slight shift in focus:

nad morzem – by the sea, at the seaside (could be on the beach, on a promenade, in a seaside town).
na plaży – specifically on the beach.

So:

Latem lubię siedzieć nad morzem…
– I like being somewhere at the seaside (not necessarily literally sitting on sand).
Latem lubię siedzieć na plaży…
– I like sitting on the beach.

Both are natural; choose based on what you want to emphasize.

What is the difference between patrzeć and oglądać in a context like this? Could I say oglądać fale?

You could say oglądać fale, but there is a nuance:

patrzeć (na coś) – to look (at something), to direct your eyes at it. Focus on the act of looking.
- Lubię patrzeć na fale. – I like looking at/watching the waves.
oglądać (coś) – to watch, view, inspect; often more active, attentive watching, like watching a film, a show, or examining something.

In the context of relaxing by the sea:

Lubię patrzeć na fale. – sounds very natural and slightly more poetic.
Lubię oglądać fale. – also possible, maybe a bit more neutral/less common for this specific image.

Both are grammatically correct; patrzeć (na fale) is the more idiomatic choice here.