Learn Hardcore Croatian: Kad zapisujem naglaske u bilježnicu, lakše vidim razliku u izgovoru. - When I write the accents in the notebook, I more easily see the difference in pronunciation.

Both mean when and are grammatically the same.
In everyday speech people use kad more often; kada can sound a bit more formal or more emphatic, but in this sentence they are fully interchangeable:

Kad zapisujem naglaske… = Kada zapisujem naglaske…

So you can safely treat them as the same word for now.

What exactly is zapisujem and how is it different from zapišem / zapisati?

Zapisujem is the 1st person singular present of zapisivati (to write down, to record), an imperfective verb.

zapisivati (imperfective) – focuses on the ongoing / repeated action
- (ja) zapisujem = I am writing down / I (usually) write down
zapisati (perfective) – focuses on a single, completed action
- (ja) zapišem = I will write down / I (manage to) write down (once)

In this sentence, the speaker describes a habit or a regular strategy, so Croatian prefers the imperfective:

Kad zapisujem naglaske… = When I (regularly) write down the accents…

If you said Kad zapišem naglaske…, it would sound more like When I (have) written the accents down (on that particular occasion)…

What form is naglaske? Why not naglasci?

The base noun is naglasak (accent, stress mark).

naglasci = nominative plural (used for the subject)
naglaske = accusative plural (used for the direct object)

In the sentence, naglaske is the thing you are writing down, so it has to be in the accusative:

(Ja) zapisujem naglaske. – I write (down) the accents.

Very simplified paradigm:

Nominative sg: naglasak
Accusative sg: naglasak
Nominative pl: naglasci
Accusative pl: naglaske

So naglaske is just “accents” as a direct object.

Why is it u bilježnicu and not u bilježnici?

In Croatian, u can take:

Accusative → movement into something
Locative → being in something (no movement)

Here we have u bilježnicu (accusative), because the idea is “I write (the accents) into the notebook” – the accents are being put there.

Compare:

Pišem u bilježnicu. – I write (into) a notebook.
Pišem u bilježnici. – Grammatically possible, but would mean something like “I am writing in the notebook” with focus on location; in practice, people overwhelmingly say u bilježnicu with pisati / zapisivati.

So: “putting something into” → u + accusative (bilježnicu).

Why does bilježnica become bilježnicu?

Bilježnica (notebook) is a regular feminine -a noun.

For feminine singular nouns like this:

Nominative: bilježnica – subject
Accusative: bilježnicu – direct object, or after u when there is movement into

Here it’s the object of the preposition phrase u bilježnicu, so it has to be in the accusative singular: bilježnicu.

What is lakše exactly? Is it an adjective or an adverb?

In this sentence, lakše functions as an adverb, meaning more easily / easier.

Base forms:

Adjective: lak – easy, light
Adverb: lako – easily

Comparative:

Adjective: lakši (m.), lakša (f.), lakše (n.) – “easier”
Adverb: lakše – “more easily, easier”

Croatian typically uses the neuter singular form (lakše) as the adverb:

Pišem lakše. – I write more easily.
Lakše vidim razliku. – I see the difference more easily.

So here lakše describes how you see the difference: you see it more easily.

Which cases are razliku and izgovoru, and why are those forms used?

razliku comes from razlika (difference) and is in the accusative singular. It is the direct object of vidim (“I see”):
- Vidim razliku. – I see a/the difference.
izgovoru comes from izgovor (pronunciation) and is in the locative singular after u:
- u izgovoru – in pronunciation

Here, u is used with locative because it expresses a static “in” (not movement into something):

razliku u izgovoru = “the difference in pronunciation”

So the structure is: (I) see (what?) → razliku (accusative) (where / in what?) → u izgovoru (locative).

Could it also be razlike u izgovoru (plural)? Would that change the meaning?

Yes, razlike u izgovoru is grammatically correct, but it changes the meaning:

razliku u izgovoru – the difference in pronunciation (seen as one difference)
razlike u izgovoru – the differences in pronunciation (several differences)

The sentence as given suggests that writing accents helps you clearly see the difference (singular) between two ways of pronouncing something. If the idea were that you see various differences, you could say:

…lakše vidim razlike u izgovoru.

Why is there a comma after bilježnicu?

The sentence has two clauses:

Kad zapisujem naglaske u bilježnicu, – subordinate clause (time)
lakše vidim razliku u izgovoru. – main clause

In standard Croatian, a subordinate clause introduced by kad is normally separated from the main clause with a comma.

You could also switch the order:

Lakše vidim razliku u izgovoru, kad zapisujem naglaske u bilježnicu.

There is still a comma between the two clauses.

Can I change the word order, for example: Kad u bilježnicu zapisujem naglaske… or Razliku u izgovoru lakše vidim…?

Yes. Croatian word order is relatively flexible, and both of those variants are possible.

Examples with slightly different emphasis:

Kad u bilježnicu zapisujem naglaske, lakše vidim razliku u izgovoru.
– perfectly fine; mild emphasis on in the notebook.
Razliku u izgovoru lakše vidim kad zapisujem naglaske u bilježnicu.
– puts razliku u izgovoru at the front, emphasizing the difference.

The basic neutral order is the original sentence, but moving phrases around is common and usually changes focus, not grammar. Just remember that clitic pronouns (like je, ga, mi, se) must follow the second-position rules, which don’t appear in this particular sentence.

Why are both verbs in the present tense? Could it be something like “When I wrote them down, I saw the difference more easily”?

In Croatian, kad + present is typically used for general, repeated, or habitual situations:

Kad zapisujem naglaske…, lakše vidim…
– Whenever I write the accents down, I see the difference more easily. (general habit)

If you want to talk about a specific past situation, you would use the past tense in both clauses:

Kad sam zapisivao naglaske u bilježnicu, lakše sam vidio razliku u izgovoru.
(m.)
Kad sam zapisivala naglaske u bilježnicu, lakše sam vidjela razliku u izgovoru.
(f.)

So the present tense here matches the idea of a regular learning strategy, not a one-time event.

What is the nuance of naglasak here? Is it “accent” like a foreign accent, or “stress” on syllables?

Naglasak can mean different things depending on context:

linguistic stress / accent on syllables
a written accent mark in Croatian textbooks
sometimes: accent in the sense of “foreign accent” (how someone sounds)

In Kad zapisujem naglaske u bilježnicu, the natural interpretation is:

naglaske = stress marks / accent marks (or the stressed syllables) that you are noting down to distinguish pronunciation.

If you meant a foreign accent in general (e.g. “He has a strong French accent”), you might say:

Ima jak francuski naglasak.