Learn Hardcore Croatian: Teško mi je učiti hrvatski navečer. - It is hard for me to study Croatian in the evening.

Using ja here would be wrong, because ja is nominative (subject form), and the structure needs a dative pronoun: mi (to me), ti (to you), mu (to him), etc.

What is the grammatical role of teško in this sentence?

Teško is a predicative adjective/adverb meaning “hard / difficult”, used impersonally.

The core structure is:

Teško je – “(It) is hard”

There is no real “it” like in English; Croatian uses a neuter predicative (teško) + 3rd person singular neuter of “to be” (je) to describe a general state:

Lako je učiti hrvatski. – It is easy to study Croatian.
Teško je raditi noću. – It is hard to work at night.

When you add the dative pronoun mi, it specifies for whom it is hard:

Teško mi je… – It is hard for me…

Why is je needed? Could I say Teško mi učiti hrvatski navečer?

You need je because this is a copular sentence (“X is Y”) with biti (“to be”).

Standard structure:

Teško mi je učiti… – Literally: “Hard to me is to study…”

Leaving out je (Teško mi učiti hrvatski navečer) sounds ungrammatical in standard Croatian. In fast informal speech some people may drop je in certain contexts, but as a learner you should always include it here.

Why is učiti in the infinitive and not učim?

In Croatian, after expressions like:

teško je (it is hard)
lako je (it is easy)
dobro je (it is good)

you normally use the infinitive to express what action is hard/easy/good:

Teško mi je učiti hrvatski. – It’s hard for me to study Croatian.
Lako mi je pričati s njom. – It’s easy for me to talk with her.
Dobro je jesti povrće. – It’s good to eat vegetables.

Saying *Teško mi je učim hrvatski is wrong, because učim is a finite verb (1st person singular present), and this construction needs the infinitive (učiti).

Does učiti mean “to learn” or “to teach”?

Učiti can mean both “to learn” and “to teach”, depending on context and subject:

Učim hrvatski. – I am learning Croatian.
Učim djecu hrvatski. – I teach children Croatian.

In your sentence:

Teško mi je učiti hrvatski navečer. the subject is implicitly “I” (from context, and from mi), and the object is hrvatski, so it clearly means:
“It’s hard for me to learn/study Croatian in the evening.”

Why is hrvatski in that form, and not hrvatski jezik or something else?

Hrvatski here is an adjective used as a noun, and it’s in the accusative singular masculine:

infinitive verb: učiti – to learn/study
direct object (what?): hrvatski – Croatian (the language)

Fully explicit you could say:

učiti hrvatski jezik – to study the Croatian language

But in everyday speech Croatians very often drop jezik and just say:

učiti hrvatski
govorim hrvatski – I speak Croatian
znam engleski – I know English

So hrvatski here behaves like a masculine noun in the accusative, even though it is morphologically an adjective.

What case is hrvatski in, and why?

Hrvatski is in the accusative case, because it is the direct object of the verb učiti (“to learn, to study”).

Pattern:

(Koga? Što?) učiti hrvatski – to study Croatian
učiti matematiku – to study math
učiti povijest – to study history

So: učiti + accusative.

What does navečer mean exactly, and how is it different from similar words?

Navečer is an adverb meaning “in the evening / during the evening (in general)”.

Compare:

večer – evening (noun)
navečer – in the evening (habitually / generally: evenings)
u večer – literally “into the evening” (rare; usually part of a more specific time phrase)
uvečer – a variant form meaning almost the same as navečer; navečer is more common in standard usage.

In your sentence:

navečer = “in the evenings / at night (in the evening times)” in a habitual or general sense:
- “It’s hard for me to study Croatian in the evening (in general).”

Can navečer be placed somewhere else in the sentence?

Yes, Croatian word order is relatively flexible, especially for adverbs like navečer. All of these are grammatical, with slightly different emphasis:

Teško mi je učiti hrvatski navečer.
Neutral; slight focus on the time at the end.
Navečer mi je teško učiti hrvatski.
Emphasis on in the evening (contrast with other times).
Teško mi je navečer učiti hrvatski.
Emphasis on the combination evening + studying Croatian.

The basic meanings are the same; word order mainly affects which part is highlighted.

Could I say Meni je teško učiti hrvatski navečer instead of Teško mi je…?

Yes, but the emphasis changes.

Teško mi je učiti hrvatski navečer.
Neutral statement: it’s hard for me.
Meni je teško učiti hrvatski navečer.
Meni is the stressed (full) dative form of ja, and puts extra emphasis on me, often contrasting with someone else:
- Meni je teško, ali njemu nije. – It’s hard for me, but not for him.

So both are correct; use mi for neutral statements, meni when you want to stress “for me (in particular)”.

Why is it not Teško je meni učiti hrvatski navečer with meni after je?

You can say Teško je meni učiti hrvatski navečer, but it sounds a bit more marked / emphatic / literary, like you’re really focusing on “for me”.

The usual neutral spoken pattern uses the clitic form (mi) in the second position:

Teško mi je učiti…

Clitic pronouns (mi, ti, mu, joj, im, nam) typically want to be very early in the sentence, usually the second “slot”. Using full meni breaks this clitic pattern and makes the pronoun more strongly emphasized:

Teško je meni… – “It is hard for me (as opposed to someone else).”

What is the actual subject of the verb je in this sentence?

Syntactically, this is an impersonal construction. There is no concrete noun subject like “it” in English.

The structure is:

Teško je učiti hrvatski navečer. – It is hard to study Croatian in the evening.

You can think of the infinitive phrase učiti hrvatski navečer as the thing being described, but in Croatian grammar this is often treated as an impersonal predicate with an infinitive, rather than as a normal “subject”.

That’s why:

teško is in neuter,
je is 3rd person singular neuter,
and there is a dative experiencer mi (“to me / for me”).

Is there a difference between Teško mi je učiti hrvatski navečer and Ja teško učim hrvatski navečer?

Yes, there is a nuance difference.

Teško mi je učiti hrvatski navečer.
- Focuses on your feeling / experience: “It is hard for me (to do this).”
- Very natural when talking about difficulty or how something feels.
Ja teško učim hrvatski navečer.
- Literally: “I learn Croatian with difficulty in the evening.”
- Grammatically possible, but sounds less natural and a bit awkward in everyday speech.
- It suggests you do it, but not easily; the structure is less idiomatic than the “Teško mi je + infinitive” pattern.

For talking about how hard something is for you, the standard idiomatic way is:
Teško mi je + infinitive.