Learn Hardcore Croatian: Ovaj zadatak je teži, ali ga razumijem. - This task is harder, but I understand it.

ovaj zadatak – this task (near me, the speaker)
taj zadatak – that task (near you, the listener, or previously mentioned)
onaj zadatak – that task over there / that task (far from both)

So the sentence Ovaj zadatak je teži, ali ga razumijem is specifically about this particular task, not just any task.

Why is zadatak in the form zadatak, not something like zadataka or zadatku? What case is it?

In Ovaj zadatak je teži, ali ga razumijem, the word zadatak is in the nominative singular, because it is the subject of the sentence.

ovaj zadatak = this task (subject)
verb: je (is)
complement: teži (harder)

Forms you might see in other contexts:

zadatak – nominative singular (subject)
zadatka – genitive singular (of the task)
zadatku – dative/locative singular (to/for/in/on the task)
zadatke – accusative plural (tasks – as direct object)

Here we’re just talking about this task as the thing that is harder, so nominative is used.

Why is the verb je used here? Can I leave it out like in some other languages?

Je is the 3rd person singular present form of biti (to be), so je = is.

Ovaj zadatak je teži = This task is harder.

In standard Croatian you usually keep the verb je in sentences like this.
You might sometimes hear people drop it in casual speech (e.g. Ovaj zadatak teži, ali ga razumijem), but that is not considered standard or careful Croatian. For learners, always include je.

What does teži mean here, and what is the base form of that adjective?

Teži means harder / more difficult.
The base (dictionary) form is težak = difficult, heavy.

It’s the comparative form:

težak zadatak – a difficult task
teži zadatak – a more difficult / harder task
najteži zadatak – the most difficult / the hardest task

So Ovaj zadatak je teži literally: This task is more difficult (than another one we’re comparing it to). The thing it’s compared to is understood from the context.

Why is teži masculine? Does it agree with zadatak?

Yes. In Croatian, adjectives (including comparative forms) must agree in gender, number, and case with the noun they describe.

zadatak is masculine singular, nominative
so the adjective must also be masculine singular, nominative → teži

Other possible forms with težak:

težak zadatak (masc. sg. nom.) – difficult task
teška knjiga (fem. sg. nom.) – difficult book
teško pitanje (neut. sg. nom.) – difficult question

Here: ovaj (m.sg.) zadatak (m.sg.) je teži (m.sg.) – everything agrees.

What does ali mean, and is the comma before ali necessary?

Ali means but.

The sentence has two parts (clauses):

Ovaj zadatak je teži – This task is harder
[ali] ga razumijem – but I understand it

They’re joined with ali (but). In standard Croatian punctuation, you do put a comma before ali, just like in English:

…, ali … = …, but …

So Ovaj zadatak je teži, ali ga razumijem is punctuated correctly.

What does ga mean here, and why is it before the verb razumijem instead of after it?

Ga is the unstressed object pronoun meaning him / it (masculine singular, accusative). Here it refers back to zadatak (a masculine noun):

razumijem zadatak – I understand the task
razumijem ga – I understand it / him

However, Croatian has strict rules about clitic (little unstressed word) placement. Pronouns like ga normally come before the main verb in a clause:

Ali ga razumijem. – But I understand it.
Ne ga razumijem. ❌ (wrong word order)
Ne razumijem ga. ✅ (because ne comes first, then the verb, then the pronoun)

In your sentence, ali ga razumijem is the natural and correct order.

Could I say ali razumijem ga instead of ali ga razumijem?

In neutral, standard Croatian, ali ga razumijem is the normal order because ga is a clitic and tends to stand immediately after the conjunction (ali) or the first stressed element in the clause.

Ali razumijem ga can occur in speech, but it usually sounds emphatic or stylistically marked, as if you’re stressing razumijem or ga:

Ali RAZUMIJEM ga. (I do understand it, actually.)
Ali razumijem GA. (I understand him/it, as opposed to someone/something else.)

As a learner, treat ali ga razumijem as the default, neutral form.

Why is there no word for I in razumijem? How do I know it means I understand?

In Croatian, subject pronouns (ja = I, ti = you, etc.) are usually dropped, because the verb ending shows the person.

The verb razumjeti (to understand) in the present tense:

(ja) razumijem – I understand
(ti) razumiješ – you understand
(on/ona/ono) razumije – he/she/it understands
(mi) razumijemo – we understand
(vi) razumijete – you (pl.) understand
(oni/one/ona) razumiju – they understand

So razumijem clearly indicates I as the subject, even without ja.
You can say Ja razumijem if you want to emphasize I, but normally you simply say razumijem.

What aspect is razumijem? Is it different from razumjeti in some way?

Razumijem is the present tense of the verb razumjeti (to understand), which is typically treated as imperfective in this meaning (ongoing or general understanding).

razumijem – I understand (now / generally)
razumio sam (masc.) / razumjela sam (fem.) – I understood (past)

In this sentence, ali ga razumijem means but I (do) understand it, as a current state or ability. Croatian aspect contrasts (perfective vs. imperfective) are not strongly visible in this particular verb in the present tense, but you should think of razumijem as expressing ongoing or general understanding, not a single completed act.

Is there any difference in meaning or style if I say Ovaj je zadatak teži, ali ga razumijem instead of Ovaj zadatak je teži, ali ga razumijem?

Both are grammatically correct and mean the same thing: This task is harder, but I understand it.

Ovaj zadatak je teži – the most neutral and common word order
Ovaj je zadatak teži – also correct, sometimes used for slight emphasis or for rhythm; it can sound a bit more formal, written, or stylistically shaped

As a learner, prefer Ovaj zadatak je teži, ali ga razumijem as your default. The alternative (Ovaj je zadatak teži) is useful to recognize and can sound more natural in certain contexts, but it’s not necessary for basic communication.