Learn Hardcore Polish: Gdy za mocno opalam się na plaży, wieczorem jest mi zimno i marznę pod małym kocem. - When I sunbathe too much on the beach, in the evening I feel cold and I freeze under a small blanket.

gdy – a bit more formal/literary, often used in written language and in more “bookish” style. Very natural in this kind of general statement.
kiedy – more neutral/colloquial; very common in everyday speech.

You could say:

Gdy za mocno opalam się na plaży, …
Kiedy za mocno opalam się na plaży, …

Both are correct. The difference here is mostly style, not grammar or meaning.

Why is opalam się in the present tense if English uses a kind of general “when I sunbathe too much” idea?

Polish uses the present tense for habitual/general truths, just like English:

Gdy za mocno opalam się na plaży
= When I (tend to) sunbathe too much on the beach

This doesn’t mean “I’m sunbathing right now”, but rather “whenever I do this, in general”.

If you wanted a specific future situation, you’d use the future tense:

Gdy za mocno się opalę na plaży, wieczorem będzie mi zimno.
When/If I sunbathe too much on the beach (this time), I will be cold in the evening.

What does za mocno mean exactly? How is it different from just mocno?

mocno = strongly, hard, a lot
za mocno = too strongly, too much (excess)

Here za is not the preposition behind; it’s an intensifier meaning too (much).

You could say:

Mocno się opalam. – I’m suntanning a lot.
Za mocno się opalam. – I’m suntanning too much. (more than is good for me)

You might also see zbyt mocno with the same meaning, slightly more formal or “careful” in tone.

Why is it opalam się and not just opalam? What does the reflexive się do here?

Opalać się is the normal way to say “to sunbathe / to tan”.

opalać się – to sunbathe, to get a tan (reflexive verb)
opalać kogoś/coś – to tan/singe something or someone (transitive)

So:

Opalam się na plaży. – I sunbathe on the beach.
Opalam drewno. – I am charring/tanning wood. (different meaning!)

In short: you need się here, because opalać się is a different verb from opalać.

Why is it na plaży and not w plaży? What case is plaży?

Polish uses different prepositions for places:

na
- locative often means “on / at” a surface or open area:
- na plaży – on the beach
- na stadionie – at the stadium
- na boisku – on the pitch
w
- locative usually means “in, inside”:
- w sklepie – in the shop
- w pokoju – in the room

You’re on a beach, not inside it, so Polish uses na.

plaży is the locative singular of plaża (feminine):

nominative: plaża (the base form)
locative: plaży (after na, w, etc. for location)

What is wieczorem exactly? Why is there no preposition like “w wieczór”?

Wieczorem is the instrumental singular of wieczór (evening) used adverbially to mean “in the evening”.

Polish often uses bare instrumental forms of time nouns to say “in the X”:

rano – in the morning (from ranek)
po południu – in the afternoon
wieczorem – in the evening
nocą – at night

You almost never say w wieczór; that sounds unnatural.
So:

Wieczorem jest mi zimno. – In the evening I am cold.

How does jest mi zimno work? Why not jestem zimny for “I’m cold”?

jest mi zimno is the natural way to say “I am cold” (I feel cold).

Structure:

jest – is
mi – “to me” (dative of ja = I)
zimno – “cold” (used adverbially/impersonally here)

Literally: “It is cold to me.”

This pattern is very common for physical sensations:

jest mi zimno – I’m cold
jest mi ciepło – I’m warm
jest mi gorąco – I’m hot
jest mi smutno – I feel sad

jestem zimny literally means “I am cold (as a cold object / a cold person)” – like your body is cold to the touch, or figuratively you are emotionally cold. It’s not used for “I feel cold” in this everyday sense.

Why do we have both jest mi zimno and marznę? Don’t they mean almost the same thing?

They overlap, but they aren’t identical:

jest mi zimno – I am (feeling) cold
marznę – I’m freezing / I’m getting really cold (stronger, more physical)

Using both together emphasizes the discomfort:

Wieczorem jest mi zimno i marznę…
≈ In the evening I’m cold and I’m (really) freezing…

It’s a bit like saying in English: “I’m cold and I’m freezing under a small blanket.” Slight repetition, but it adds intensity and sounds natural.

What exactly does marznę mean, and how is it different from zamarzam?

Both come from the root marznąć (to freeze, to get very cold):

marznę – I am freezing / I’m getting very cold
- ongoing process of getting cold, feeling very chilly
zamarzam – more like I am freezing solid / I’m freezing up
- can be literal (water freezing) or very strong figurative “I’m freezing to death”

So in this sentence:

marznę pod małym kocem – I’m freezing under a small blanket (I feel really cold, but I’m not turning into ice)

zamarzam pod kocem would sound overly dramatic or like you are literally freezing.

Why is it pod małym kocem and not something like pod mały koc?

The preposition pod usually takes:

instrumental when it means location:
- pod stołem – under the table
- pod małym kocem – under a small blanket
accusative when it can mean “to under / towards under” (direction), in some contexts:
- e.g. Pies pobiegł pod stół. – The dog ran under the table.

Here we are talking about location (where you are freezing), so instrumental is used.

Forms:

koc – nominative singular (masculine)
kocem – instrumental singular
mały koc (nom.) → małym kocem (instr.)

So pod małym kocem = under a small blanket.

Why is there no “a” / “the” before małym kocem? Can Polish show that it’s “a small blanket” and not “the small blanket”?

Polish has no articles (a, an, the). Context does the job.

pod małym kocem can mean “under a small blanket” or “under the small blanket” depending on the situation.

If you need to be very specific, you use other devices:

demonstratives: pod tym małym kocem – under this small blanket
possessives: pod moim małym kocem – under my small blanket

But in a general statement about what happens when you sunbathe too much, just pod małym kocem is perfect.

Why is there no “ja” (I) in opalam się and marznę? When would you add it?

Polish usually drops subject pronouns (ja, ty, on, etc.) because the verb ending shows the person:

opalam się – the -am ending already shows 1st person singular (“I”).
marznę – the -ę ending also shows 1st person singular.

You add ja only for emphasis or contrast:

To nie ty marzniesz, tylko ja marznę pod małym kocem.
It’s not you who’s freezing, it’s me who’s freezing under a small blanket.

In your sentence, there’s no contrast, so ja would sound unnecessary.

Can I move się in opalam się? For example, say za mocno się opalam na plaży?

Yes. Polish allows some flexibility with the placement of się.

Both are correct:

Gdy za mocno opalam się na plaży, …
Gdy za mocno się opalam na plaży, …

Native speakers often prefer za mocno się opalam in everyday speech, but both word orders sound natural. The meaning doesn’t change; it’s just small stylistic/rythmic differences.

Is the word order in the second part fixed? Could I say Wieczorem marznę i jest mi zimno pod małym kocem?

Yes, you can change the word order; Polish is quite flexible:

Wieczorem jest mi zimno i marznę pod małym kocem.
Wieczorem marznę pod małym kocem i jest mi zimno.

Both are grammatical. The original order:

starts with the more neutral “I’m cold” (jest mi zimno)
then intensifies to “and I’m freezing” (i marznę)

If you swap them, you slightly change the rhythm and emphasis, but the basic meaning remains the same.