Learn Hardcore Croatian: Kad stalno učiš, lice ti ponekad izgleda umorno. - When you study all the time, your face sometimes looks tired.

It doesn’t describe “right now only”; it describes a repeated, habitual situation.
So the present učiš here is the generic/habitual present, which is natural in both languages.

Why is there no explicit ti (“you”) before učiš?

In Croatian, personal subject pronouns (ja, ti, on, ona, mi, vi, oni…) are usually dropped, because the verb ending shows the person:

učiš already tells us it’s “you (sg.) study”.

You can say Ti stalno učiš for emphasis (“You are always studying”), but the neutral, normal version just says stalno učiš (or kad stalno učiš).

What exactly does stalno mean here, and how is it different from uvijek?

Stalno means “constantly, all the time, continually”.
Uvijek means “always”.

In practice:

stalno often carries a nuance of “too often / so often it’s annoying or excessive”, especially in speech:
- On stalno priča. = He keeps talking (all the time / it’s annoying).
uvijek is more neutral “always”:
- On uvijek priča. = He always talks (as a regular fact).

Here, Kad stalno učiš suggests you’re studying so much / very often, not just a neutral “always”.

Can I change the word order and say Kad učiš stalno instead of Kad stalno učiš?

You could say Kad učiš stalno, but it’s less natural in this particular sentence.

Kad stalno učiš… (adverb stalno right before the verb) is the most natural and typical:
“When you constantly study…”

Putting stalno right before učiš clearly shows that the way you study is constant/continuous.
Kad učiš stalno sounds more marked, like you’re stressing “at the times when you study, it’s always”—not wrong, but less smooth here.

Why is it lice ti and not tvoje lice for “your face”?

Croatian often uses a dative pronoun to show a possessor, especially with body parts and things closely connected to a person.

tvoje lice = your face (using the possessive adjective tvoje)
lice ti = literally “the face to you”, but idiomatically “your face”

So:

Lice ti izgleda umorno. ≈ Tvoje lice izgleda umorno. = Your face looks tired.

The version with a dative pronoun is very natural and common:

Ruke mi se tresu. = My hands are shaking.
Glava mu boli. (colloq.) = His head hurts.

In careful standard language you’ll also see Glava ga boli (accusative), but dative of the possessor as here (lice ti) is everyday, idiomatic.

What is the function of ti in lice ti ponekad izgleda umorno? Is it an indirect object?

Grammatically, ti is dative singular of “ti” (you), but here it mainly shows possession:

lice ti = “the face (that belongs) to you” = your face.

So in this sentence, ti isn’t a normal indirect object like in “I gave you a book”; it’s a dative of the possessor, a common Slavic structure:

Oči mi su crvene. = My eyes are red.
Novčanik mu je pao. = His wallet fell (lit. wallet to-him fell).

Why is the order lice ti and not ti lice?

Croatian clitic pronouns (like mi, ti, mu, joj, im, se) have fixed positioning rules:

They usually appear after the first stressed word or phrase in the clause.

In lice ti ponekad izgleda umorno:

lice is the first stressed word.
The clitic ti must come right after it.

That gives: Lice ti ponekad izgleda umorno.

You cannot say Ti lice ponekad izgleda umorno; that’s wrong word order.
You can say Tvoje lice ponekad izgleda umorno. (without the clitic).

Could we just say Izgledaš umorno instead of Lice ti izgleda umorno? What’s the difference?

Yes, you can say:

Izgledaš umorno. = You look tired.

Difference in nuance:

Lice ti izgleda umorno. = Literally “Your face looks tired.”
Focuses on the appearance of the face specifically.
Izgledaš umorno. = “You look tired.”
The whole person appears tired; more general comment.

Both can often be used in the same situation, but the original sentence explicitly talks about the face.

Why is the adjective umorno and not umoran or umorna?

The adjective must agree with the grammatical gender and number of the noun lice:

lice is neuter singular.
So the predicate adjective must also be neuter singular: umorno.

Patterns:

muškarac je umoran (masc. sg.)
žena je umorna (fem. sg.)
lice je umorno (neut. sg.)

Therefore:

Lice ti ponekad izgleda umorno.
not umoran or umorna.

Why is there a comma after Kad stalno učiš?

Because Kad stalno učiš is a subordinate clause (a dependent “when”-clause) placed before the main clause:

Subordinate clause: Kad stalno učiš
Main clause: lice ti ponekad izgleda umorno.

In Croatian, when a subordinate clause comes first, there is normally a comma before the main clause:

Kad dođeš kući, nazovi me.
Ako pada kiša, ostajemo doma.

What is the role of ponekad in the sentence, and can it be moved?

Ponekad means “sometimes”.

In lice ti ponekad izgleda umorno, the neutral word order is:

[Subject] lice ti
[Adverb] ponekad
[Verb] izgleda
[Predicate] umorno

You can move ponekad somewhat:

Lice ti izgleda ponekad umorno. – possible, but sounds a bit marked.
Ponekad ti lice izgleda umorno. – also possible, with stronger emphasis on sometimes.

The original Lice ti ponekad izgleda umorno is a very natural default:
“Your face sometimes looks tired.”

Why is the verb izgleda and not izgleda kao (like in “looks like”)?

Croatian izgledati has two common patterns:

izgledati + adjective
- izgleda umorno = looks tired
- izgleda lijepo = looks beautiful
izgledati kao + noun/pronoun
- izgleda kao učiteljica = she looks like a teacher
- izgleda kao on = he looks like him

In your sentence, umorno is an adjective, so the correct construction is izgledati + adjective, without kao:

Lice ti ponekad izgleda umorno.
not izgleda kao umorno.

Is there any aspect issue with učiš? Why not naučiš?

Yes, aspect matters:

učiti = imperfective (“to study / to be studying, to learn in progress or habit”)
naučiti = perfective (“to learn/finish learning, to have learned”)

In “When you constantly study”, we want a continuous/habitual action, so we must use the imperfective:

Kad stalno učiš… ✅

A perfective verb like naučiš would express a single completed act of learning and does not fit the idea of ongoing constant studying:

Kad stalno naučiš ❌ (ungrammatical / doesn’t make sense here).