Learn Hardcore Polish: Dzieci szybko się nudzą na wykładzie, jeśli nic nie widać na ekranie. - Children quickly get bored in the lecture if nothing can be seen on the screen.

In this sentence, we clearly mean that the children themselves are bored, not that they are boring someone, so się is required:
Dzieci szybko się nudzą…

Can I change the word order to Dzieci szybko nudzą się? Is there any difference?

Yes, you can say Dzieci szybko nudzą się. Both:

Dzieci szybko się nudzą…
Dzieci szybko nudzą się…

are grammatically correct and natural.

Notes:

The “default” placement of się in modern Polish is usually right after the first stressed element of the verb phrase (here: szybko się nudzą), so the original version sounds slightly more neutral/typical.
Putting się after the verb (nudzą się) is also fine and common, especially in speech.

There is no real change in meaning here, just a slight shift in rhythm.

Could I drop się and just say Dzieci szybko nudzą na wykładzie?

No, not with the intended meaning.

Dzieci szybko się nudzą = Children quickly get bored.
Dzieci szybko nudzą (without się) would be understood as Children quickly bore (someone) – they are the ones causing boredom in others.

So in this sentence, się is not optional; it is essential to express that the children are bored, not boring.

What tense/aspect is nudzą here, and does it mean “are getting bored” or “get bored”?

Nudzą is:

present tense
imperfective aspect
3rd person plural of nudzić się

In context, it expresses a habitual/general situation, not a one-time event. So in English it is best read as:

Children *get bored quickly in lectures…* (a general truth)
Rather than a specific ongoing event:
Children *are getting bored quickly in the lecture…* (one occasion)

Polish present tense of an imperfective verb often covers both “are doing” and “do (habitually)”, so context decides. Here, the conditional clause jeśli… gives it a general, habitual meaning.

Why is it na wykładzie and not na wykład?

Both structures exist, but they mean different things:

na wykład (accusative) – to a lecture (movement towards):
- Idę na wykład. – I’m going to a lecture.
na wykładzie (locative) – at/in a lecture (being somewhere):
- Jestem na wykładzie. – I’m at a lecture.

In your sentence we are talking about what happens while the lecture is going on, so we use the static location form:

Dzieci szybko się nudzą na wykładzie…
Children quickly get bored during/at a lecture…

What case is wykładzie, and how is it formed from wykład?

Wykładzie is the locative singular of wykład (lecture).

For many masculine nouns ending in a consonant, the locative singular ending is -e or -ie. Here:

nominative: wykład
locative (after na, w, etc.): na wykładzie

You also see -ie in forms like:

w sklepie – in the shop (from sklep)
na stole – on the table (locative of stół is irregular, but pattern similar in function)

What’s the difference between wykład and lekcja?

Both are translated as lesson or class sometimes, but they’re used in different contexts:

wykład
- typically a lecture, especially at a university or academic context
- suggests one person speaking to a bigger audience
- e.g. wykład z historii sztuki – an art history lecture
lekcja
- a lesson/class at school, usually in primary/secondary education
- also used for private lessons
- e.g. lekcja angielskiego – an English lesson

In a university-style context, na wykładzie is the natural phrase. In a school classroom context, you’d more often say na lekcji.

Why do we use na wykładzie instead of w wykładzie? Both mean “in/on”, right?

Na and w don’t overlap freely:

na is used for:
- surfaces: na stole – on the table
- events/activities: na koncercie, na wykładzie, na lekcji
- many institutions/locations: na uniwersytecie, na poczcie
w is used more for:
- physical interiors: w pokoju – in the room
- “inside” something: w pudełku – in the box

A lecture is treated as an event, so Polish uses na wykładzie (at a lecture), not w wykładzie. Using w wykładzie would sound wrong to a native speaker.

What exactly is going on in nic nie widać? Why two negatives?

Polish uses negative concord: when the sentence is negative and you have words like nic (nothing), nikt (nobody), they must be combined with nie on the verb.

So:

Nic nie widać. – literally: Nothing not is-visible.
Natural English: Nothing can be seen. / You can’t see anything.

If you said:

Nic widać. – this is simply ungrammatical.
Nie widać. – means [One] can’t see (it) or It’s not visible (without specifying “nothing”).

So in a fully explicit negative like “nothing can be seen”, Polish requires the “double negative” nic nie widać. It’s correct and standard.

What is widać grammatically? Is it a verb?

Yes, widać functions like an impersonal verb meaning roughly:

is visible / can be seen

Key points:

It has no explicit subject:
Nie widać nic. – Nothing can be seen. (literally “it is not seen anything”)
It’s related in meaning to widzieć (to see), but you don’t conjugate it like widzę, widzisz, etc.
You normally just use widać, nie widać.

Examples:

Widać góry. – You can see the mountains. / The mountains are visible.
Nic nie widać na ekranie. – Nothing can be seen on the screen.

In your sentence, nic nie widać is an impersonal construction: there is no “subject” in the English sense.

Could I say jeśli nic nie jest widoczne na ekranie instead of nic nie widać na ekranie?

You can, but it sounds more formal and less natural in everyday speech.

nic nie widać na ekranie
– very common, neutral, colloquial-friendly; what people actually say.
nic nie jest widoczne na ekranie
– grammatically correct, but more formal / bookish; it sounds like a technical or written description.

In conversation or natural narrative, nic nie widać na ekranie is strongly preferred.

What case is ekranie, and why do we use na ekranie here?

Ekranie is the locative singular of ekran (screen).

As with na wykładzie, the preposition na + locative means “on” in the sense of being on a surface or visible on a display:

na ekranie – on the screen
na tablicy – on the board
na ścianie – on the wall

So nic nie widać na ekranie means “nothing can be seen on the screen,” not “inside” the screen. W ekranie would sound wrong here.

Does dzieci always take a plural verb like nudzą?

Yes. Dzieci is grammatically plural.

Singular vs plural:

singular: dziecko – a child
- Dziecko się nudzi. – The child is getting bored.
plural: dzieci – children
- Dzieci się nudzą. – The children are getting bored.

Polish verbs in 3rd person plural don’t change form based on masculine/feminine/non‑masculine‑personal the way adjectives and past tense do; they just use one plural present form: nudzą. So dzieci nudzą się / dzieci się nudzą is always with plural nudzą.

Can I replace jeśli with gdy or kiedy here? Do they mean the same?

In your sentence, you can say:

…jeśli nic nie widać na ekranie.
…gdy nic nie widać na ekranie.
…kiedy nic nie widać na ekranie.

All are possible and understandable, but there are nuances:

jeśli – focuses on condition (“if”), often used in more logical/conditional statements.
gdy – more like “when/whenever,” slightly more literary, but common.
kiedy – “when,” very common in everyday speech, often temporal but can also be conditional in context.

In this specific sentence:

jeśli highlights the conditional idea: if it happens that nothing can be seen on the screen, then children get bored (a rule).
gdy/kiedy shift a bit more toward “whenever/when that happens,” but the overall meaning remains very similar.

All three are acceptable; jeśli fits well for a clear “if–then” statement.