Learn Hardcore Czech: Můj bratr se směje, když slyší starý příběh, který zná z dětství. - My brother laughs when he hears an old story that he knows from childhood.

Without se, směje on its own is incomplete and sounds wrong in standard Czech. So you always need se with smát se (similar to how in English you always need “to” in “to laugh at something” – you can’t just say “I laugh it”).

Why is the reflexive se placed after bratr and not immediately after Můj? Why Můj bratr se směje and not Můj se bratr směje?

Czech has a rule that short pronouns like se, si, mi, ti, ho often go in the so‑called “second position” in the clause.

Můj bratr se směje
Here, Můj bratr is treated as one unit (a noun phrase), and the clitic se then comes after that whole unit. So it’s still effectively in “second position”.

Můj se bratr směje sounds ungrammatical to native speakers. The natural positions in this sentence are:

Můj bratr se směje. – neutral, normal
Bratr se směje. – also normal (without Můj)

What is the difference between když and kdy, and why is když used here?

když = when as a conjunction introducing a clause (like when he hears…). It links two actions or situations.
kdy = when as a question word (direct or indirect question: When? / I don’t know when…).

In the sentence Můj bratr se směje, když slyší…:

když slyší starý příběh = when(ever) he hears the old story
This is not a question, so we use když, not kdy.

Examples:

Kdy přijdeš? – When are you coming? (question)
Přijdu, když budu mít čas. – I’ll come when I have time. (conjunction)

Why is it slyší and not uslyší here?

Czech distinguishes imperfective and perfective verbs:

slyšet (imperfective) – to hear (in general, ongoing, repeated)
uslyšet (perfective) – to hear (once, to catch/notice a sound at a moment)

In když slyší starý příběh, we are talking about a habitual/repeated situation: every time he hears that story, he laughs. For that, Czech prefers the imperfective slyšet → slyší.

If you said:

Můj bratr se usměje, když uslyší starý příběh.
that would emphasize a more one‑time, momentary reaction at the instant he hears it.

What case is starý příběh, and how can I tell?

Starý příběh is in the accusative singular, because it is the direct object of the verb slyší (he hears what? → the old story).

Clues:

The base noun is příběh (masculine inanimate).
In masculine inanimate, nominative and accusative forms are often the same (příběh → příběh).
The adjective starý agrees in gender, number, and case. For masculine inanimate accusative singular, the form of starý is also starý.

So even though the form looks like nominative, functionally it’s accusative as the object of slyší.

Why is it Můj bratr and not Mého bratra?

Because Můj bratr is the subject of the sentence, and subjects are in the nominative case.

Můj bratr se směje.
- Můj – nominative masculine singular (possessive pronoun)
- bratr – nominative masculine singular (noun)

Mého bratra would be genitive or accusative, e.g.:

Vidím mého bratra. – I see my brother. (direct object → accusative)
Nemám mého bratra rád. – I don’t like my brother. (also accusative)

But in your sentence, můj bratr is the one doing the laughing, so nominative is required.

In který zná z dětství, what exactly does který refer to, and what case is it?

Který is a relative pronoun meaning which / that, and it refers back to příběh.

příběh – masculine inanimate singular
so we also use který in masculine singular

Function in the clause:

zná co? – (he) knows what? → který
So který is the direct object of zná, which means it is in the accusative case.

For masculine inanimate, nominative and accusative look the same:

Nominative: který
Accusative: který (same form)

So it looks nominative, but grammatically here it functions as accusative.

Why is it zná and not ví? Don’t both mean “knows”?

Czech distinguishes two common verbs for to know:

znát – to know someone/something (to be familiar with it)
- Znám ten příběh. – I know that story.
vědět – to know a fact, information, answer
- Vím, že je starý. – I know that it is old.

In který zná z dětství, he knows/familiar with the story from childhood, so znát is correct:

zná ten příběh – he knows that story

Ví ten příběh is wrong. You’d use ví, for example, in:

Ví, kdo mu ten příběh vyprávěl. – He knows who told him the story.

What does z dětství literally mean, and what case is dětství in?

z dětství literally means from (his) childhood.

z is a preposition that usually takes the genitive case (z + genitive = from/out of).
dětství is a neuter noun meaning childhood.

For neuter nouns ending in -í (like dětství), the nominative singular and genitive singular forms are the same in form:

Nominative: dětství – childhood
Genitive: dětství – of childhood

So the word dětství doesn’t change its shape, but in the phrase z dětství it functions as genitive because of the preposition z.

Is se směje “laughs” or “smiles”? How is that different from usmívá se?

smát se → se směje = to laugh
- Můj bratr se směje. – My brother laughs.
usmívat se → se usmívá = to smile (continuous/imperfective)

The sentence Můj bratr se směje, když slyší starý příběh… clearly describes laughing, not just smiling.

If you wanted to say he smiles when he hears the story, you would use:

Můj bratr se usmívá, když slyší starý příběh.

Why is there a comma before když in this sentence?

In Czech, a comma is normally used before conjunctions that introduce a subordinate clause, such as když, protože, že, aby etc.

Můj bratr se směje, když slyší starý příběh…
- Main clause: Můj bratr se směje
- Subordinate clause: když slyší starý příběh, který zná z dětství

The comma separates the main clause (the main message) from the subordinate když‑clause (the condition/time when it happens). This is very similar to English punctuation in My brother laughs when he hears…

Who is the subject of slyší and zná? There is no on (“he”) written.

In Czech, the subject pronoun (já, ty, on, ona, my, vy, oni) is often dropped because the verb ending already tells you who the subject is.

slyší – 3rd person singular (he/she/it hears)
zná – 3rd person singular (he/she/it knows)

From context, we understand that the subject of both verbs is můj bratr:

Můj bratr se směje, když (on) slyší starý příběh, který (on) zná z dětství.

Adding on would be grammatically possible but stylistically unnecessary and usually avoided:

Můj bratr se směje, když on slyší… – sounds heavy or emphatic; normally you wouldn’t say it.

Could I say když slyší ten starý příběh instead of když slyší starý příběh? Is there any difference?

Yes, you can say:

když slyší ten starý příběh

The difference is mostly in nuance:

starý příběh – an old story; could be understood more generally, or as “that old story” if context is clear.
ten starý příběh – that old story; more specific and definite, pointing to a particular well‑known story.

In normal conversation, both are possible here. If you want to stress that it’s that specific story he knows from childhood, ten starý příběh sounds more explicitly definite.