Learn Hardcore Polish: Kiedy jestem zmęczony, wtedy wolę zostać w domu. - When I am tired, then I prefer to stay at home.

Zmęczony is an adjective meaning tired in the masculine singular form.
In Polish, adjectives agree in gender, number, and case with the noun or pronoun they describe.

Here, the implied subject is ja (I), and the default assumption is a male speaker, so you use zmęczony.
If a woman were speaking, she would normally say:

Kiedy jestem zmęczona, wtedy wolę zostać w domu.

So:

zmęczony – masculine (I, he)
zmęczona – feminine (I, she)
zmęczeni – masculine personal plural (we – a group with at least one man)
zmęczone – non-masculine plural (we – all women or things/animals)

Do I really need both kiedy and wtedy? Can I drop wtedy?

You don’t have to use both. The most natural, everyday version would usually be:

Kiedy jestem zmęczony, wolę zostać w domu.

Here, kiedy = when, and wtedy = then. Using both together literally mirrors English “When I am tired, then I prefer to stay at home.”

In modern Polish, wtedy is often optional and can sound a bit more emphatic or slightly more formal/bookish in this kind of sentence.
So:

With wtedy: adds a small emphasis on the consequence (then I prefer …).
Without wtedy: completely correct and very common.

Can I change the word order to Wolę zostać w domu, kiedy jestem zmęczony?

Yes, that’s perfectly correct and very natural:

Wolę zostać w domu, kiedy jestem zmęczony.

In Polish, both orders are fine:

Kiedy jestem zmęczony, wolę zostać w domu.
Wolę zostać w domu, kiedy jestem zmęczony.

The meaning doesn’t really change. The only difference is what you focus on first:

Starting with kiedy… highlights the condition.
Starting with wolę… highlights the preference.

The comma is required either way, because you’re joining a subordinate clause (kiedy…) with the main clause (wolę…).

Why is it wolę zostać and not something like wolę zostaję?

In Polish, wolę (I prefer) is followed by an infinitive verb, just like English “I prefer to stay”:

wolę zostać – I prefer to stay

You cannot say wolę zostaję.
Zostaję is a finite form (I am staying), and you don’t put two finite verbs together in this structure.

So the pattern is:

wolę
- infinitive (zostać, iść, czytać, etc.)
  - Wolę zostać w domu. – I prefer to stay at home.
  - Wolę iść do kina. – I prefer to go to the cinema.

What’s the difference between zostać and zostawać?

Both mean roughly to stay / to remain, but they differ in aspect:

zostać – perfective: focuses on the result, the decision/act of staying.
zostawać – imperfective: focuses on the ongoing process or repeated action.

With wolę, you normally use the infinitive that describes the intended result, so zostać is more natural:

wolę zostać w domu – I prefer to (choose to) stay at home.

Using wolę zostawać w domu would sound unusual here; it might suggest a more habitual preference like I prefer being a person who (usually) stays at home, but even then Poles would almost always still say wolę zostać w domu for this sentence.

Why is it w domu and not something like w dom or do domu?

Domu is the locative case of dom (house, home). The preposition w (in) usually takes the locative when it means in / inside:

w domu – in (the) house, at home

You can’t say w dom; that’s ungrammatical.
Do domu is a different expression:

do domu – to home / to the house (movement towards)
- Idę do domu. – I’m going home.
- Jestem w domu. – I am at home.

In your sentence, you’re not moving; you’re staying in a place, so w domu is correct.

Why is there no ja in the sentence? Shouldn’t it be Kiedy ja jestem zmęczony…?

In Polish, the personal pronoun (like ja = I) is usually dropped, because the verb ending already shows the person:

jestem = I am
wolę = I prefer

Poles only add ja for emphasis or contrast:

Kiedy ja jestem zmęczony, wtedy wolę zostać w domu, a on wychodzi.
When *I am tired, I prefer to stay at home, but he goes out.*

In a neutral statement, Kiedy jestem zmęczony, wtedy wolę zostać w domu is the normal form.

Could you omit jestem and just say Kiedy zmęczony, wolę zostać w domu?

In standard Polish, you need the verb jestem:

Kiedy jestem zmęczony, wolę zostać w domu.

Dropping jestem (When tired, I prefer…) is not natural in normal, correct Polish.
You might sometimes see things like:

Gdy zmęczony, nie pracuje dobrze.

But that sounds either incomplete, stylistically odd, or like a headline/note, not like normal spoken or written Polish. For clear, correct language, keep jestem.

What’s the difference between kiedy, gdy, and jak in this kind of sentence?

In this sentence, you can say:

Kiedy jestem zmęczony, wolę zostać w domu.
Gdy jestem zmęczony, wolę zostać w domu.

Kiedy and gdy often overlap and can both mean when.
Rough tendencies:

kiedy – very common, neutral.
gdy – slightly more formal or literary in some contexts, but still widely used in speech.

Jak can also mean “when” in colloquial speech:

Jak jestem zmęczony, wolę zostać w domu.

This sounds informal/conversational. In writing or more careful speech, kiedy (or gdy) is safer.

Why do we use wolę, not lubię?

Both verbs are about liking, but they’re not the same:

lubię – I like (something in general)
wolę – I prefer (something rather than something else)

In this sentence, you’re talking about a choice / preference in a situation (being tired), not just a general liking. So wolę is more precise:

Kiedy jestem zmęczony, wolę zostać w domu.
When I’m tired, I *prefer to stay at home.*

Using lubię would change the meaning:

Kiedy jestem zmęczony, lubię zostać w domu.
This sounds more like When I’m tired, I (just) like staying at home – less about a clear preference, more about enjoyment.

Is zostać a reflexive verb? Do I need się?

Zostać here is not reflexive, so you do not use się.

zostać – to stay / to remain / to become (depending on context)
- Wolę zostać w domu. – I prefer to stay at home.
- Został lekarzem. – He became a doctor.

Reflexive verbs use się, e.g.:

uczyć się – to learn
bać się – to be afraid

But zostać się is not used in standard Polish in this meaning. So the correct form in your sentence is simply zostać.

Why is there a comma before wtedy wolę zostać w domu?

Polish uses commas to separate subordinate clauses from the main clause.
Here:

Kiedy jestem zmęczony – subordinate clause (introduced by kiedy)
(wtedy) wolę zostać w domu – main clause

So you need a comma:

Kiedy jestem zmęczony, wtedy wolę zostać w domu.

The same rule applies if you reverse the order:

Wolę zostać w domu, kiedy jestem zmęczony.

How do you pronounce zmęczony and what’s going on with the letter ę?

Zmęczony is pronounced roughly like: zmen-CHO-nih, with stress on -cho-:

z – like z in zoo
mę – nasal vowel, similar to French “en” in “enfant” or “an” in “chant” (but shorter)
cz – like ch in “chocolate”, but a bit harder
o – like o in “not” (British-ish)
ny – like ni in “onion” (but shorter, more central vowel)

The letter ę is a nasal vowel. Before cz, it sounds nasal but not followed by a clear n/m sound. Don’t pronounce it as “e-n”; think of it as one nasalized vowel.