Ovaj zadatak je dobro objašnjen.

Ovaj means this.

It appears as ovaj because it has to agree with zadatak, which is:

• masculine
• singular
• nominative

So:

• ovaj zadatak = this task/exercise
• ovo would be neuter, so it would not match zadatak

For example:

• ovaj zadatak = this task
• ova knjiga = this book
• ovo pitanje = this question

Croatian demonstratives change form depending on gender, number, and case.

Croatian does not have articles like English the or a/an.

So zadatak can mean:

• a task
• the task

The exact meaning depends on context.

In this sentence, ovaj already makes it specific:

• ovaj zadatak = this task

So there is no need for a separate word meaning the.

Because it is the subject of the sentence, and subjects normally appear in the nominative case.

Here:

• ovaj zadatak = the thing being talked about
• je dobro objašnjen = is well explained

So zadatak stays in its basic dictionary form, which here is nominative singular.

Je is the 3rd person singular present form of biti, meaning to be.

So in this sentence it means is:

• Ovaj zadatak je dobro objašnjen.
• This task is well explained.

It links the subject ovaj zadatak with objašnjen.

It is originally a past passive participle, a verb-derived form, but in sentences like this it behaves a lot like an adjective.

It agrees with the noun:

• masculine singular: objašnjen
• feminine singular: objašnjena
• neuter singular: objašnjeno
• plural: objašnjeni / objašnjene / objašnjena, depending on gender

So in:

• Ovaj zadatak je dobro objašnjen

objašnjen matches zadatak, which is masculine singular.

You can think of the whole phrase je dobro objašnjen as is well explained.

Because objašnjen must agree with zadatak.

Zadatak is:

• masculine
• singular

So the correct form is objašnjen.

Compare:

• Ovaj zadatak je dobro objašnjen. = masculine singular
• Ova lekcija je dobro objašnjena. = feminine singular
• Ovo pitanje je dobro objašnjeno. = neuter singular

This kind of agreement is very important in Croatian.

Because dobro is an adverb, and here it means well.

It modifies objašnjen in the sense of well explained, not good explained.

So:

• dobro = well
• dobar / dobra / dobro = good

In this sentence you need the adverb:

• dobro objašnjen = well explained

This is similar to English:

• correct: well explained
• not: good explained

Yes. Croatian word order is more flexible than English word order.

The neutral, straightforward order is:

• Ovaj zadatak je dobro objašnjen.

But you may also hear:

• Ovaj je zadatak dobro objašnjen.
• Dobro je objašnjen ovaj zadatak.

These versions are grammatically possible, but they may sound more marked or put emphasis on different parts of the sentence.

For a learner, the original sentence is the safest neutral pattern.

Yes, it is essentially a passive or passive-like construction.

It focuses on the task, not on who explained it.

• Ovaj zadatak je dobro objašnjen. = This task is well explained.
• active version: Netko je dobro objasnio ovaj zadatak. = Someone explained this task well.

Croatian often uses active sentences when the doer matters, but passive-style sentences like this are very common when the doer is unknown, unimportant, or obvious from context.

It comes from the verb objasniti, meaning to explain.

The form objašnjen is the passive participle, meaning roughly explained.

So:

• objasniti = to explain
• objašnjen = explained

The stem changes a bit, so this is not something you can always guess perfectly at first sight. It is a good idea to learn it as a pair:

• objasniti → objašnjen

These are all demonstratives, but they point to different things.

• ovaj = this, near me / the speaker
• taj = that, near you or already mentioned
• onaj = that over there, farther away

So:

• ovaj zadatak = this task
• taj zadatak = that task
• onaj zadatak = that task over there

In your sentence, ovaj shows that the speaker means this particular task.

It can mean several related things, depending on context.

Common possibilities are:

• task
• exercise
• problem
• assignment

For example:

• in a textbook, zadatak often means exercise
• in school, it can mean assignment
• in general use, it can mean task

So the best English translation depends on where the sentence appears.