Learn Hardcore Croatian: Nije lako učiti hrvatski. - It is not easy to learn Croatian.

nije – is not / it is not (3rd person singular of biti – to be, in the negative)
lako – easy, here functioning like easy / easily
učiti – to learn / to study (infinitive)
hrvatski – Croatian (literally Croatian [language])

So the structure is basically: is‑not easy to‑learn Croatian, which in natural English becomes It is not easy to learn Croatian.

Why is there no word for it at the beginning, like It is not easy…?

Croatian usually does not use a dummy subject like English it in sentences about the weather, time, or in general statements:

English: It is not easy to learn Croatian.
Croatian: Nije lako učiti hrvatski. (literally Is‑not easy to‑learn Croatian)

The “subject” of the sentence is actually the whole infinitive phrase učiti hrvatski (to learn Croatian). Because that phrase comes later, Croatian doesn’t need an extra it at the start. The verb nije can start the sentence by itself.

Why is nije written as one word and not ne je?

Nije is the standard contracted negative form of je (3rd person singular of biti – to be).

je = is
ne
- je → nije = is not

You almost never see ne je in normal Croatian; it fuses into nije. The same happens with some other forms of biti, for example:

ne + sam → nisam (I am not)
ne + smo → nismo (we are not)
ne + su → nisu (they are not)

So nije is just the correct negative form, not two separate words.

Is lako an adjective or an adverb here? Why not lak?

In Nije lako učiti hrvatski, lako behaves like an adverb: it describes how something is (how easy it is) in a neutral, general way.

lak / laka / lako are adjective forms (easy for masculine / feminine / neuter nouns).
lako is also the adverb (easily / in an easy way).

After je / nije plus an infinitive (učiti), Croatian very often uses the neuter/adverb form like lako, teško, zanimljivo:

Lako je učiti hrvatski. – It is easy to learn Croatian.
Teško je učiti hrvatski. – It is hard to learn Croatian.

Using lak here (Nije lak učiti hrvatski) would be ungrammatical.

Why is učiti (infinitive) used instead of a conjugated form like učim?

Here učiti is used as an infinitive because we are talking about the activity in general: to learn Croatian.

The infinitive (učiti) works like the English to learn in It is not easy to learn Croatian.
If you use učim (I learn / I am learning), you must have a clear subject (ja učim hrvatski – I am learning Croatian).

Compare:

Nije lako učiti hrvatski. – It is not easy to learn Croatian (in general).
Nije mi lako učiti hrvatski. – It is not easy for me to learn Croatian. (still infinitive)
Ne učim lako hrvatski. – I do not learn Croatian easily. (conjugated verb učim)

Why is hrvatski in that form? What case is it, and where is the noun jezik?

Hrvatski here is the accusative singular masculine form of the adjective hrvatski (Croatian) used as a noun, meaning Croatian (language). The object of učiti (to learn) is in the accusative:

učiti što? – učiti hrvatski (learn what? learn Croatian)

Croatian often drops jezik (language) when talking about languages:

učiti hrvatski (jezik) – learn Croatian (language)
učiti engleski (jezik) – learn English (language)

Because the masculine adjective hrvatski looks the same in nominative and accusative for an inanimate noun, it doesn’t change shape, but grammatically here it is the accusative object.

Could you also say učiti hrvatski jezik, and is there any difference?

Yes, učiti hrvatski jezik is perfectly correct and means the same thing: to learn the Croatian language.

učiti hrvatski – shorter, very common, sounds natural and colloquial.
učiti hrvatski jezik – a bit more explicit or formal; you might see it in textbooks, course titles, or official contexts.

In everyday speech, most people would just say učiti hrvatski.

Can the word order change, like Učiti hrvatski nije lako?

Yes, Croatian allows flexible word order, and Učiti hrvatski nije lako is also correct. Common variants include:

Nije lako učiti hrvatski. – neutral, very natural.
Učiti hrvatski nije lako. – slight emphasis on the activity učiti hrvatski.

The meaning stays essentially the same; the difference is mostly in emphasis and rhythm. All of these remain more or less neutral and acceptable.

What is the difference between učiti and naučiti in a sentence like this?

Učiti is imperfective – it focuses on the process of learning.
Naučiti is perfective – it focuses on the result (to succeed in learning, to have learned).

So:

Nije lako učiti hrvatski. – It is not easy to learn Croatian (the process of studying it is not easy).
Nije lako naučiti hrvatski. – It is not easy to (fully) learn Croatian / to get it learned (reaching the end result is not easy).

Both are correct; the first talks more about ongoing learning, the second about actually achieving knowledge/mastery.

How would I say the positive version: It is easy to learn Croatian?

You just remove the negation:

Lako je učiti hrvatski. – It is easy to learn Croatian.

Note the typical positive order: Lako je…, not Je lako….
In the negative, Nije lako učiti hrvatski, the ne is built into nije, and nije can naturally stand first.