Moja nećakinja je danas donijela svjedodžbu kući i ponosno je pokazala baki.

Yes:

• moja = my
• nećakinja = niece
• je = auxiliary verb used to form the past tense
• danas = today
• donijela = brought
• svjedodžbu = report card / school report / certificate, in the accusative
• kući = home
• i = and
• ponosno = proudly
• je = here it can be understood as it referring back to svjedodžbu; see below for the ambiguity
• pokazala = showed
• baki = to grandma, in the dative

So the structure is basically:

My niece + has/brought + the report card home today + and proudly showed it to grandma.

Because Croatian does not have articles like English.

English needs words like a and the:

• a niece
• the report card

Croatian usually leaves that information to context:

• nećakinja can mean a niece or the niece
• svjedodžbu can mean a report card or the report card

If Croatian speakers want to be more specific, they can use other words, such as:

• ta svjedodžba = that report card
• ova svjedodžba = this report card

But in an ordinary sentence like this, no article is needed.

Because moja has to agree with nećakinja.

The noun nećakinja is:

• feminine
• singular
• nominative

So the possessive adjective moj changes to match it:

• moj nećak = my nephew
• moja nećakinja = my niece

This agreement in gender, number, and case is very important in Croatian.

Croatian past tense is usually made with:

• a present-tense form of biti = to be
• plus the l-participle of the main verb

Here:

• je = auxiliary, 3rd person singular
• donijela = past participle of donijeti

So:

• ona je donijela = she brought

The ending -la shows that the subject is feminine singular.

Compare:

• on je donio = he brought
• ona je donijela = she brought
• oni su donijeli = they brought

The same feminine pattern appears in pokazala.

This is one of the trickiest parts.

In i ponosno je pokazala baki, the je is easy to notice because it looks exactly like the auxiliary je.

With the meaning you were given, it is best understood as the unstressed pronoun it, referring to svjedodžbu.

So the sense is:

• ... and proudly showed it to grandma

Why is that possible? Because in Croatian, when two past-tense verbs share the same subject, the repeated auxiliary is often omitted.

So the full underlying idea is something like:

• Moja nećakinja je danas donijela svjedodžbu kući i ponosno ju je pokazala baki.

But Croatian often shortens this.

Also, forms like je and ju can both come up in this kind of context, and many speakers prefer ju when they want to avoid ambiguity.

Because svjedodžbu is the accusative singular form.

The base form is:

• svjedodžba = nominative singular

But here it is the direct object of donijela:

• she brought what? → svjedodžbu

So the noun changes case:

• svjedodžba = subject form
• svjedodžbu = direct-object form

This is a very common feminine noun pattern:

• knjiga → knjigu
• torba → torbu
• svjedodžba → svjedodžbu

Because baki is in the dative singular.

With pokazati, the person who receives or is shown something is usually in the dative:

• pokazati komu? = to show to whom?

So:

• baka = grandmother, nominative
• baki = to grandmother, dative

Here:

• pokazala baki = showed to grandma

This is the same idea as:

• dala mami = gave to mom
• rekla učitelju = said to the teacher
• poslala prijatelju = sent to a friend

Kući is a very common Croatian way to say home, especially with verbs of motion.

So:

• ići kući = to go home
• doći kući = to come home
• donijeti kući = to bring home

It does not literally work exactly like the noun kuća in every context. It is a set expression that learners should treat as a normal phrase.

Useful comparison:

• kući = home
• kod kuće = at home

So:

• Ona ide kući. = She is going home.
• Ona je kod kuće. = She is at home.

Often yes, depending on region and style.

Both can relate to home:

• kući
• doma

But they are not always identical in feel.

In many contexts:

• Idem kući. = I’m going home.
• Idem doma. = I’m going home.

In this sentence, kući is very standard and natural:

• donijela svjedodžbu kući

Some speakers might also say:

• donijela svjedodžbu doma

That is also common in many places, but kući is a very safe standard choice for learners.

Because ponosno is an adverb, while ponosna is an adjective.

Here it describes how she showed the report card:

• pokazala kako? = how did she show it?
• ponosno = proudly

So:

• ponosna = proud, feminine adjective
• ponosno = proudly, adverb

Compare:

• Moja nećakinja je ponosna. = My niece is proud.
• Moja nećakinja je ponosno pokazala svjedodžbu. = My niece proudly showed the report card.

Because the sentence describes completed actions.

The verbs here are perfective:

• donijeti = to bring, as a completed act
• pokazati = to show, as a completed act

That fits the situation:

1. she brought the report card home
2. she showed it to grandma

Both happened as finished events.

If you used imperfective verbs, the meaning would change:

• donosila suggests repeated, ongoing, or background action
• pokazivala suggests repeated or prolonged showing

So for a single finished event, the perfective forms are the natural choice.

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

This sentence is natural and neutral, but other versions are possible:

• Danas je moja nećakinja donijela svjedodžbu kući.
• Moja je nećakinja danas donijela svjedodžbu kući.
• Svjedodžbu je moja nećakinja danas donijela kući.

The basic meaning stays the same, but the emphasis changes.

One important thing, though: short unstressed words such as je, ga, ju, se, mi, ti often have to appear in an early clitic position in the clause. So Croatian word order is flexible, but not completely free.