Learn Hardcore Polish: Nad morzem szybko marznę w wodzie, ale na plaży jest mi gorąco. - By the sea I quickly get cold in the water, but on the beach I feel hot.

morze is the basic form (nominative singular, the sea).
With nad expressing location (over / by / at something), Polish uses the instrumental case:
- morze → morzem (instrumental singular)

So:

nad morzem = by/at the sea, at the seaside (literally above the sea), correct here
nad morze would be accusative, used for movement towards the sea:
- Jadę nad morze. – I’m going to the seaside.

You need nad morzem because the sentence describes being located there, not moving there.

What is the difference between nad morzem, na morzu, and przy morzu?

They all involve the sea, but they picture different locations:

nad morzem
- Literally above the sea, but idiomatically by the sea / at the seaside.
- Normally means you are on land, near the coast (e.g. in a seaside town, on the beach, in a hotel by the sea).
- Very common in the sense of on holiday at the seaside.
na morzu
- Literally on the sea.
- Used when you are on the water: on a boat, ship, or otherwise out at sea.
- State (location) version of:
  - wypłynęliśmy na morze – we sailed out to sea.
przy morzu
- Literally near/next to the sea.
- Grammatically correct but much less common in everyday speech.
- Would typically sound like a more literal or slightly bookish way of saying near the sea (e.g. Miasto leży przy morzu – The town lies by the sea).

In everyday conversation about vacations, nad morzem is the standard phrase: Jestem nad morzem – I’m at the seaside.

What verb is marznę from, and what does it mean exactly?

Marznę is the 1st person singular present tense of the verb marznąć.

Infinitive: marznąć – to get cold, to freeze (feel very cold)
Present tense:
- ja marznę – I am getting cold / I freeze (quickly)
- ty marzniesz
- on/ona/ono marznie
- my marzniemy
- wy marzniecie
- oni/one marzną

In this sentence, szybko marznę w wodzie means I get cold quickly in the water (I start feeling cold quickly), not that the water itself is freezing.

The perfective counterpart is zmarznąć – to get (fully) cold / to freeze (once, completely):

Zmarzłem / Zmarzłam w wodzie. – I got very cold in the water.

What is the difference between marznę and jest mi zimno?

Both express feeling cold, but they focus on different things:

marznę
- Verb meaning I am getting cold / I’m freezing (up).
- Suggests a process or tendency: you become cold, often fairly quickly or strongly.
- Szybko marznę w wodzie. – I start feeling cold quickly in the water.
jest mi zimno
- Literally: it is cold to me → I feel cold.
- Describes a state: you are (already) cold.
- Jest mi zimno w wodzie. – I feel cold in the water.

You could combine them in context:

Szybko marznę w wodzie, więc po chwili jest mi bardzo zimno.
I get cold quickly in the water, so after a while I feel very cold.

Why is it w wodzie and not w woda or w wodę?

W (in) is a preposition and needs the noun in a particular case.

The noun is woda (water), feminine.

With w, the case depends on meaning:

Location (where?) → locative case
- woda → wodzie (locative singular)
- w wodzie = in the water (where you are)
In the sentence szybko marznę w wodzie, you are located in the water, so it uses w wodzie.
Motion into (where to?) → accusative case
- woda → wodę (accusative singular)
- wchodzę w wodę – I (am) go(ing) into the water

So:

w wodzie – in the water (static location)
w wodę – into the water (movement)

Why is it na plaży instead of na plażę or just plaża?

Again, it is the preposition na that controls the case.

Noun: plaża (beach), feminine.

With na:

Location (where?) → locative case
- plaża → plaży (locative singular)
- na plaży – on the beach (where you are)
Motion onto (where to?) → accusative case
- plaża → plażę (accusative singular)
- Idę na plażę. – I’m going to the beach.

In the sentence, the meaning is on the beach it is hot for me, describing location, so you need na plaży.

What does the structure jest mi gorąco mean, and why is mi in the dative?

Jest mi gorąco literally means it is hot to me, idiomatically I am hot / I feel hot.

Structure:

jest – it is (3rd person singular of być – to be)
mi – dative of ja (to me, for me), unstressed form
gorąco – adverb, hot / warm (as a feeling)

Polish often uses a dative experiencer with adjectives/adverbs for physical states:

jest mi gorąco – I am hot
jest mi zimno – I am cold
jest mi smutno – I am sad
jest mi dobrze – I feel good

You cannot say mam gorąco for I am hot; that sounds wrong. You need jest mi gorąco or simply gorąco mi.

Why do we use mi and not mnie in jest mi gorąco?

Both mi and mnie are dative forms of ja, but:

mi – unstressed, clitic form
mnie – stressed or used after prepositions

In short:

In normal, neutral sentences like jest mi gorąco, you use mi.
Use mnie to emphasize or after a preposition:
- To mnie jest gorąco, nie jemu. – It’s me who is hot, not him. (emphasis)
- Do mnie jest gorąco is not idiomatic; the typical pattern here doesn’t use a preposition, so you still say jest mi gorąco.

Jest mnie gorąco is ungrammatical. You either say:

jest mi gorąco (neutral), or
to mnie jest gorąco / mnie jest gorąco (with emphasis on me).

What part of speech are szybko and gorąco here?

Both are adverbs in this sentence:

szybko – from the adjective szybki (quick, fast), means quickly / fast
- szybko marznę – I get cold quickly.
gorąco – from the adjective gorący (hot), means hotly / it is hot as a feeling
- jest mi gorąco – I feel hot.

As adjectives, they would be:

szybki:
- szybka woda – fast water (current)
gorący:
- gorąca woda – hot water

But because they describe how you get cold or how you feel, they are adverbs here.

Why is there no ja (I) in szybko marznę w wodzie?

Polish usually drops subject pronouns when the verb ending clearly shows the person.

marznę already tells us the subject is ja (1st person singular).
Adding ja would sound marked or emphatic:
- Ja szybko marznę w wodzie – I (as opposed to others) get cold quickly in the water.

So the natural, neutral version is simply szybko marznę w wodzie without ja.

Is the comma before ale in ..., ale na plaży jest mi gorąco obligatory?

Yes. In Polish, a comma before ale is almost always obligatory when it connects two clauses.

Here we have two clauses:

Nad morzem szybko marznę w wodzie
(ale) na plaży jest mi gorąco

They are joined by ale (but), so we must write:

Nad morzem szybko marznę w wodzie, ale na plaży jest mi gorąco.

Can the word order in ale na plaży jest mi gorąco be changed?

Yes. Polish word order is flexible; different orders change emphasis slightly but stay grammatical. Examples:

Ale na plaży jest mi gorąco. – neutral, light emphasis on on the beach.
Ale jest mi gorąco na plaży. – emphasizes more the feeling hot, then adds on the beach.
Ale na plaży gorąco mi jest. – more expressive or poetic.
Ale gorąco mi na plaży. – very natural in speech, strong focus on gorąco mi (I’m so hot).

All are understandable; the original is a neutral, standard choice.

How do the prepositions nad, w, and na work with cases for location vs movement?

In this sentence we only have location, but it is useful to see the pattern:

nad (over, by, above)
- Location (where?) → instrumental
  - nad morzem – by/at the sea
- Movement (where to?) → accusative
  - jadę nad morze – I’m going to the seaside
w (in, into)
- Location (where?) → locative
  - w wodzie – in the water
- Movement (where to?) → accusative
  - wchodzę w wodę – I’m walking into the water
na (on, onto, to)
- Location (where?) → locative
  - na plaży – on the beach
- Movement (where to?) → accusative
  - idę na plażę – I’m going to the beach

In the given sentence all three prepositional phrases express where something happens, so they use instrumental/locative (depending on the preposition), not accusative.