Ovaj zadatak je najlakši, pa ću ga završiti za deset minuta.

Because ovaj must agree with zadatak in gender, number, and case.

• zadatak is masculine singular
• it is the subject of the sentence, so it is in the nominative
• the matching form of this is ovaj

So:

• ovaj zadatak = this task / this exercise
• ova knjiga = this book (feminine)
• ovo pitanje = this question (neuter)

It is in the nominative singular, because it is the subject of the sentence.

The basic dictionary form is zadatak, and here it stays in that form because the sentence is saying something about the task:

• Ovaj zadatak je najlakši = This task is the easiest

If it were the object, the form might change depending on the noun and its gender, but here it is the subject.

je is the 3rd person singular present of biti = to be.

So je najlakši literally means is the easiest.

This is a normal Serbian pattern:

• On je visok = He is tall
• Knjiga je zanimljiva = The book is interesting
• Zadatak je najlakši = The task is the easiest

najlakši is the superlative form of lak = easy / light.

The pattern is:

• lak = easy
• lakši = easier
• najlakši = easiest

So Serbian often forms the superlative by adding naj- to the comparative form.

In this sentence, najlakši agrees with zadatak, so it is masculine singular.

This is just how the comparative of lak is formed. Many Serbian adjectives do not make the comparative in a completely predictable way from the English learner’s point of view.

For lak, the comparative is:

• lak → lakši

Then the superlative is:

• lakši → najlakši

So this is something worth learning as a vocabulary pattern rather than trying to build it from English logic.

Here pa means something like:

• so
• therefore
• and so
• then

In this sentence, it connects the two ideas:

• This task is the easiest
• so I’ll finish it in ten minutes

It is very common in everyday Serbian and often sounds natural and conversational.

ću is part of the future tense.

Serbian commonly forms the future with:

• a short form of hteti = to want / will
• plus the infinitive

Here:

• ću = I will
• završiti = finish

So:

• ću završiti = I will finish

In the full form, it comes from ja ću.

Both patterns exist in Serbian, but they are used in slightly different ways.

You can often see:

• završiću = I will finish

But in this sentence you have:

• pa ću ga završiti

This is also completely correct and very natural. The future auxiliary ću is a clitic, so it tends to appear near the beginning of its clause.

So these are both possible:

• Završiću ga za deset minuta
• Ja ću ga završiti za deset minuta
• Pa ću ga završiti za deset minuta

ga is the clitic object pronoun meaning it or him, depending on context.

Here it means it, and it refers back to zadatak.

So:

• završiti ga = finish it

Serbian usually does not repeat the noun if it is already clear. Instead of saying završiću zadatak, the sentence says završiću ga because the task has already been mentioned.

Here ga is in the accusative, because it is the direct object of završiti.

The verb završiti takes a direct object:

• završiti zadatak = to finish a task
• završiti ga = to finish it

So ga is the accusative form used for that object.

Because ga is a clitic pronoun, and Serbian clitics have special word-order rules.

Clitics usually go early in the clause, often in the second position area, not where English would put them.

So Serbian prefers:

• pa ću ga završiti

rather than an English-like order such as placing ga later.

A useful thing to remember is that short forms like ću, ga, je, se, and similar words often gather near the beginning of the clause.

Because završiti is perfective, and the sentence is talking about a completed result: finishing the task.

Serbian aspect matters a lot:

• završiti = to finish, to complete
• završavati = to be finishing / to finish repeatedly / to be in the process of finishing

Since the speaker means I will complete it, the perfective završiti is the natural choice.

This also fits well with za deset minuta, which gives the time needed to reach completion.

Here za deset minuta means in ten minutes, in the sense of within ten minutes or it will take ten minutes to finish.

This is a very important Serbian pattern:

• za + accusative can express the time needed to complete something

Examples:

• Uradiću to za sat vremena = I’ll do it in an hour
• Pročitaću knjigu za dva dana = I’ll read the book in two days

So in your sentence:

• završiću ga za deset minuta = I’ll finish it in ten minutes

It does not mean for ten minutes.

Because after numbers like pet, šest, deset, dvadeset, Serbian normally uses a special form of the noun that corresponds to the genitive plural.

So:

• jedan minut
• dva minuta
• pet minuta
• deset minuta

That is why you see deset minuta, not deset minuti or deset minut.

Not completely. Serbian word order is fairly flexible, but clitics such as ću and ga have strong placement rules.

This sentence is a very natural order:

• Ovaj zadatak je najlakši, pa ću ga završiti za deset minuta.

You could also say:

• Ovaj zadatak je najlakši, pa ga ću... → this is not natural / not correct
• Završiću ga za deset minuta → correct
• Ja ću ga završiti za deset minuta → correct, with extra emphasis on I

So the sentence is flexible in some ways, but clitic position is the part you have to be careful with.

Yes. You could say:

• Zadatak je najlakši, pa ću ga završiti za deset minuta.

That would still be grammatical.

Adding ovaj makes it more specific:

• ovaj zadatak = this particular task

So ovaj helps point to a specific task, maybe one among several.