Najlakši zadatak danas bio je onaj koji sam radila kod kuće.

Najlakši means the easiest.

It is the superlative form of lak = easy / light:

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

In Serbian, the superlative is usually made with naj- + the comparative form, so it is not built like English most easy.

Because najlakši has to agree with zadatak.

Zadatak is:

• masculine
• singular
• nominative here, because it is the subject of the sentence

So the adjective also appears in the masculine singular nominative form: najlakši.

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

In a school or workbook context, it often means:

• an exercise
• a problem
• a task to complete

So in this sentence it could naturally refer to something like a homework task or exercise.

Both orders can be correct in Serbian, because Serbian word order is fairly flexible.

• Najlakši zadatak danas je bio onaj...
• Najlakši zadatak danas bio je onaj...

Both are natural. The version with bio je can sound a bit more literary, stylistic, or focused on what comes after it. The basic meaning does not change.

So this is mostly a matter of word order and emphasis, not a different tense or grammar rule.

Onaj means that one.

In this sentence, bio je onaj koji... means was the one that...

So onaj helps identify a specific task:

• not just any task
• but that particular one which the speaker did at home

It is a very natural way in Serbian to say the one that...

Because together they make the structure the one that...

• onaj = that one
• koji = which / that

So:

onaj koji sam radila kod kuće
= the one that I did at home

You could sometimes rephrase the sentence without onaj, but this version is very natural and avoids repeating zadatak.

Koji is a relative pronoun. It introduces the relative clause:

koji sam radila kod kuće
= that I did at home

It refers back to onaj / zadatak.

Its gender and number match the thing it refers to:

• masculine
• singular

Its case depends on its role inside the relative clause. Here it is the object of radila, so it is in the accusative.

Because it refers to a masculine inanimate noun, the accusative form looks the same as the nominative: koji.

Yes. Radila shows that the speaker is female.

In the Serbian past tense, the main verb agrees with the subject in gender and number:

• sam radio = I did / I was doing (male speaker)
• sam radila = I did / I was doing (female speaker)

So this sentence tells us the speaker is female.

Because sam is a clitic. Serbian clitics usually go in the second position of their clause.

The relative clause begins with koji, so the clitic comes right after it:

• koji sam radila

This is the normal and expected order.

Yes, uradila could also work, but the nuance is a little different.

• radila zadatak = did / was working on the task
• uradila zadatak = did and completed the task

So:

• koji sam radila kod kuće can sound a bit more general
• koji sam uradila kod kuće stresses completion more clearly

Both can be natural, depending on what exactly you want to say.

Kod kuće is the normal Serbian expression for at home.

It means being at one’s home in a general sense.

By contrast:

• kod kuće = at home
• u kući = in the house

So u kući focuses more on being physically inside the building, while kod kuće is the idiomatic choice for at home.

It is in the genitive singular.

That is because the preposition kod requires the genitive:

• kuća = house / home
• kod kuće = at home

So kuće is the genitive form of kuća.

Because koji sam radila kod kuće is a restrictive relative clause: it identifies which one.

It is essential information:

• onaj koji sam radila kod kuće = the one that I did at home

In this kind of phrase, Serbian usually does not use a comma before koji.

A comma would be more likely if the relative clause were just extra, non-essential information.

Yes. Serbian word order is flexible, so danas can move.

For example:

• Najlakši zadatak danas bio je onaj koji sam radila kod kuće.
• Danas je najlakši zadatak bio onaj koji sam radila kod kuće.
• Najlakši zadatak bio je danas onaj koji sam radila kod kuće.

These versions are all possible, but the emphasis changes slightly. The original sentence sounds natural and emphasizes the easiest task today as the topic.