Learn Hardcore Polish: W dolinie pod górą płynie cicha rzeka. - In the valley under the mountain a quiet river flows.

Instrumental for location (where something is):
- pod górą – under the mountain (at a fixed place)
Accusative for movement towards:
- pod górę – (going) under/up the mountain / towards the mountain

In this sentence we have a static spatial relation (the valley is under the mountain), so pod uses the instrumental: pod górą.

Why is the order płynie cicha rzeka (verb before subject) instead of something like cicha rzeka płynie or rzeka płynie cicho?

Polish word order is fairly flexible. All of these are grammatically correct:

Płynie cicha rzeka.
Cicha rzeka płynie.
Rzeka płynie cicho.

The differences are mainly about information structure and emphasis, not basic grammar:

Cicha rzeka płynie – more neutral, like English “A quiet river flows.”
Płynie cicha rzeka – slightly more poetic/literary; the verb first can highlight the action (“is flowing”) and then present what it is that flows.
Rzeka płynie cicho – focuses more on how it flows (quietly).

In your sentence, the word order contributes to a more descriptive, poetic feel, common in literary or lyrical Polish.

How do I know that rzeka is the subject in this sentence?

You can tell rzeka is the subject mainly by its case and agreement with the verb:

Case
- Rzeka is in the nominative singular (dictionary form).
- Subjects are typically in the nominative.
Verb agreement
- The verb płynie is 3rd person singular.
- Rzeka is singular, so it matches.

The other nouns are in oblique cases:

w dolinie – locative (after w)
pod górą – instrumental (after pod)

Since rzeka is the only noun in nominative, and it agrees in number with płynie, it functions as the subject.

Why is it cicha rzeka and not cichy rzeka?

Cicha is the feminine form of the adjective cichy (“quiet”).

Polish adjectives have to agree in gender, number, and case with the noun they describe.

Noun: rzeka – feminine, singular, nominative
Adjective: cichy – base form (masculine singular nominative)

To match rzeka, you need the feminine nominative singular form:

masculine: cichy dom
feminine: cicha rzeka
neuter: ciche jezioro

So cicha rzeka is the correct agreement.

Why is it cicha rzeka (adjective before noun)? Can I say rzeka cicha?

Both orders are possible, but they usually carry a slightly different stylistic or emphasis nuance:

cicha rzeka – the neutral, normal way to say “a quiet river.”
rzeka cicha – more poetic, expressive, or contrastive, like “a river, [one that is] quiet.”

In everyday, neutral speech, adjectives normally go before the noun.
Putting them after the noun often sounds:

poetic/literary: rzeka cicha, góra wysoka
or contrastive: To rzeka, cicha, ale głęboka. (It’s a river, quiet but deep.)

In your sentence, cicha rzeka is the standard, neutral choice.

What tense and aspect is płynie, and what is the base verb?

Płynie is:

Present tense
3rd person singular
From the imperfective verb płynąć – to flow / to swim (when focusing on the ongoing process).

Approximate forms:

infinitive: płynąć – to flow
ja: płynę – I flow / I am flowing
ty: płyniesz
on/ona/ono: płynie
my: płyniemy
wy: płyniecie
oni/one: płyną

So płynie describes an ongoing, continuous action in the present: is flowing / flows.

Why is w followed by locative (dolinie), while pod is followed by instrumental (górą)? Is there a rule?

Yes. In Polish, each preposition is associated with one or more specific cases, often depending on whether it describes location or movement.

For these two:

w (“in”)
- Locative for location:
  - w dolinie – in the valley (static place)
- Accusative for movement into:
  - w dolinę – into the valley
pod (“under/below”)
- Instrumental for location:
  - pod górą – under the mountain (where something is)
- Accusative for movement towards/under:
  - pod górę – (going) under/up the mountain

In your sentence, both w and pod describe where something is (location), not movement, so:

w → locative: w dolinie
pod → instrumental: pod górą

Could I say Cicha rzeka płynie w dolinie pod górą instead? Does it change the meaning?

You can absolutely say:

Cicha rzeka płynie w dolinie pod górą.

The basic meaning is the same. The differences are in focus and rhythm:

Cicha rzeka płynie w dolinie pod górą.
Starts with the subject, more like neutral narration.
W dolinie pod górą płynie cicha rzeka.
Starts with the location, setting the scene, then introduces what is there. This can sound a bit more picturesque or literary, like painting a landscape.

Grammatically, both are correct and natural.

Why isn’t there any word for “a” or “the” before cicha rzeka?

Polish has no articles like English “a/an” or “the”.

The noun phrase cicha rzeka can mean:

a quiet river
the quiet river

Context (and sometimes word order or stress) tells you whether something is new information (like English “a”) or known/specific (like “the”), but no extra word is added.

So cicha rzeka covers both “a quiet river” and “the quiet river,” depending on context.

How are the special Polish letters in this sentence pronounced, like ł and ą?

Key special letters here:

ł in płynie
- Pronounced roughly like English w in “we”.
- płynie ≈ P-WY-nyeh (very approximate).
ą in górą
- Nasal vowel, somewhat like “on” in French “bon” or like “own” but nasal.
- At the end of a word, it’s often pronounced close to /ɔw̃/ or /ɔm̃/ depending on dialect.
- górą ≈ GOO-rą (with nasalized -ą).

Other notes:

rz in rzeka is usually pronounced like ż (similar to the s in “measure”).
ci in cicha before a vowel is a soft “ch” sound, like ć.

So very roughly:

W dolinie pod górą płynie cicha rzeka ≈
v do-LEE-nyeh pod GOO-rõ pwY-nyeh TSEE-kha ZHE-ka
(highly approximate, just to give you a feel).