Learn Hardcore Croatian: Volim zapisivati nove riječi u bilježnicu. - I like to write down new words in my notebook.

In Croatian, the verb ending already shows who the subject is, so the subject pronoun is usually dropped.
Volim is the 1st person singular form of voljeti, so it already means I like / I love.
You can say Ja volim zapisivati…, but that puts extra emphasis on I (as in: I like it, maybe others don’t).
In a neutral sentence, Croatians almost always just say Volim… without ja.

Why is volim followed by zapisivati (the infinitive)? In English we usually say I like doing, not I like to do.

In Croatian, verbs like voljeti (to like/love), htjeti (to want) and morati (to have to) are very often followed by the infinitive.
So Volim zapisivati… literally corresponds to I like to write down…, but in English you normally translate it as I like writing down….
Other examples:

Volim čitati. – I like reading.
Volim učiti jezike. – I like learning languages.

Using the infinitive here is the standard, natural pattern.

Why is zapisivati used instead of zapisati or pisati?

There are two main points here: meaning and aspect.

Meaning
- pisati = to write (in general).
- zapisivati / zapisati = to write something down, to note it somewhere (e.g. in a notebook, on a list).
  So zapisivati fits better because you are specifically recording words somewhere.
Aspect
- zapisivati is imperfective (focus on the process, repeated/habitual action).
- zapisati is perfective (a single completed act: write it down once, finish it).

With general likes and habits (I like doing X), Croatian normally uses the imperfective:
Volim zapisivati… = I like regularly / habitually writing new words down.
Volim zapisati… would sound strange in this general, habitual sense.

Does volim mean I like or I love here?

The verb voljeti covers both like and love, depending on context.

With people: Volim te. usually means I love you.
With activities and objects: Volim kavu., Volim čitati. is normally I like coffee, I like reading.

So in Volim zapisivati nove riječi u bilježnicu., the natural translation is I like writing down new words in a notebook.

What is going on grammatically with nove riječi? Why those endings?

Nove riječi is in the accusative plural, because it is the direct object of zapisivati (what do I like writing down? – new words).

The basic form is nova riječ = a new word.
- nova – feminine singular (nominative) of nov (new).
- riječ – feminine singular (nominative) = word.
In the sentence we need plural object, so we get:
- nove riječi = new words (accusative plural, feminine).

For feminine inanimate nouns, the accusative plural looks the same as the nominative plural, which is why you see riječi in both roles.

Why is it u bilježnicu and not u bilježnici?

The preposition u can take either accusative or locative, and the choice changes the meaning:

u + accusative = movement into something, direction
- u bilježnicu = into the notebook
u + locative = location in something
- u bilježnici = in the notebook

In Volim zapisivati nove riječi u bilježnicu., you are putting words into the notebook, so u bilježnicu (accusative) is correct.

Compare:

Idem u grad. – I am going to town. (accusative, movement)
Sam u gradu. – I am in town. (locative, location)

What form is bilježnicu, and what does bilježnica mean exactly?

Bilježnicu is accusative singular feminine of bilježnica.

Nominative: bilježnica – a notebook / exercise book.
Accusative: bilježnicu – used as the object after u (with movement) or after many verbs.

So:

Imam novu bilježnicu. – I have a new notebook.
Pišem u bilježnicu. – I write in a notebook.

It is not knjiga (book in the sense of a published book), but a personal notebook you write in.

Can I change the word order, or must it be exactly Volim zapisivati nove riječi u bilježnicu?

Croatian word order is fairly flexible, especially compared to English.
Volim zapisivati nove riječi u bilježnicu. is the neutral, most typical order, but you can move parts for emphasis, for example:

Nove riječi volim zapisivati u bilježnicu.
(emphasis on nove riječi – new words in contrast to something else)
Volim u bilježnicu zapisivati nove riječi.
(slight emphasis on u bilježnicu – in a notebook, not somewhere else)

The basic information does not change; only the focus or emphasis shifts.

How would I say this in the past? For example, I used to like writing down new words in a notebook.

You use the past tense of voljeti plus the same infinitive:

Masculine speaker: Volio sam zapisivati nove riječi u bilježnicu.
Feminine speaker: Voljela sam zapisivati nove riječi u bilježnicu.

This usually means I used to like writing down new words in a notebook or I liked writing down new words in a notebook (in some past period).

How do I make this sentence negative: I don’t like writing down new words in a notebook?

You simply add ne in front of volim:

Ne volim zapisivati nove riječi u bilježnicu. – I don’t like writing down new words in a notebook.

The rest of the sentence stays the same; the negation is carried by ne volim.

How do I pronounce the tricky parts like riječi and bilježnicu, and what about the special letters?

A few key pronunciation points:

lj in bilježnicu is a single sound, similar to the lli in million, but smoother and more palatal.
ž in bilježnicu is like s in English measure.
č in riječi is like ch in chocolate.
ije in riječi is pronounced roughly like a long ye in yes: ri-ye-chi.

So, approximately:

riječi ≈ ri-ye-chi
bilježnicu ≈ bi-lye-zh-ni-tsu

All vowels are clear and short unless marked by context; Croatian spelling is very consistent with pronunciation.