Ovaj zadatak je lak, ali sledeći je mnogo teži.

Because ovaj means this and it has to agree with the noun zadatak in gender, number, and case.

• zadatak is masculine singular
• here it is in the nominative case, because it is the subject
• so the correct form is ovaj

Compare:

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

Zadatak usually means task, exercise, or assignment, depending on context.

In school or workbook contexts, it often means:

• exercise
• problem
• task

So in this sentence, ovaj zadatak could naturally be understood as this exercise or this problem.

In Serbian, many adjectives have a short form and a long form in masculine singular nominative.

For lak:

• lak = short form
• laki = long form

In predicate position after je (is), Serbian normally uses the short form:

• Zadatak je lak. = The task is easy.

The long form is used much less often in modern standard Serbian in this kind of sentence, and would usually not be the normal choice here.

It can mean both, depending on context.

• lak kamen would mean a light stone
• lak zadatak means an easy task

So here lak clearly means easy, because it describes zadatak.

Its opposite can also vary by context:

• težak kofer = a heavy suitcase
• težak zadatak = a difficult/hard task

Because Serbian often leaves out a noun when it is already understood from context.

So:

• Ovaj zadatak je lak, ali sledeći je mnogo teži.

literally means:

• This task is easy, but the following one is much harder.

The word zadatak is omitted in the second clause because it is obvious that sledeći refers to the next task.

You could also say the fuller version:

• Ovaj zadatak je lak, ali sledeći zadatak je mnogo teži.

But that sounds less natural because it repeats information unnecessarily.

Sledeći means following or next.

In this sentence, it means:

• the next one
• the following task

It behaves like an adjective, so it agrees with the implied noun zadatak:

• masculine singular nominative: sledeći

A learner may also see the variant sljedeći, especially in ijekavian varieties.
So:

• sledeći = Ekavian
• sljedeći = Ijekavian

Both mean the same thing.

Because the sentence has two clauses joined by ali (but):

1. Ovaj zadatak je lak
2. ali sledeći je mnogo teži

Each clause has its own predicate, so je is used in both.

English sometimes also repeats the verb:

• This task is easy, but the next one is much harder.

So the Serbian structure is very natural.

Mnogo teži means much harder or far more difficult.

• teži = harder / more difficult
• mnogo = much / a lot

So:

• teži = harder
• mnogo teži = much harder

Here mnogo intensifies the comparative.

Because teži is the comparative form of težak.

Serbian comparatives are often irregular or involve stem changes.
For this adjective:

• težak = hard, difficult, heavy
• teži = harder, more difficult, heavier

Similarly:

• lak = easy, light
• lakši = easier, lighter

So teži is simply the correct comparative form to learn.

Yes. It agrees with the implied noun zadatak.

Even though zadatak is not repeated, it is still understood, so teži is masculine singular nominative, matching:

• sledeći (zadatak)
• teži (zadatak)

That is why you see:

• sledeći je mnogo teži

and not a feminine or neuter form.

Ali means but.

It connects two contrasting ideas:

• Ovaj zadatak je lak = This task is easy
• ali sledeći je mnogo teži = but the next one is much harder

So ali introduces contrast, just like but in English.

The given word order is completely natural:

• Ovaj zadatak je lak, ali sledeći je mnogo teži.

Serbian word order is somewhat flexible, but this is the most neutral and straightforward version.

You might also hear variations for emphasis, such as:

• Ovaj zadatak je lak, ali je sledeći mnogo teži.

That is possible, but the original sentence is the most standard and natural for a learner to use.

Not in the same meaning.

• lak = easy
• lakši = easier

The sentence is not comparing this task to something else in the first clause; it is simply describing it:

• This task is easy

So lak is correct there.

If you said Ovaj zadatak je lakši, that would mean:

• This task is easier

That would imply a comparison, even if the thing compared is not stated explicitly.