Oni grade novu kuću u gradu.

Yes. In Croatian, subject pronouns are often omitted because the verb ending already shows the subject.

• grade = they are building / they build
• So Grade novu kuću u gradu. is also a complete sentence.

Oni is kept when you want to:

• emphasize they
• contrast them with someone else
• make the subject especially clear

So both are correct:

• Oni grade novu kuću u gradu.
• Grade novu kuću u gradu.

Grade is the 3rd person plural present tense form of the verb graditi (to build).

Present tense of graditi:

• ja gradim = I build / am building
• ti gradiš = you build / are building
• on/ona/ono gradi = he/she/it builds / is building
• mi gradimo = we build / are building
• vi gradite = you plural/formal build / are building
• oni/one/ona grade = they build / are building

So oni grade means they build or they are building, depending on context.

Because kuću is the direct object of the verb grade.

They are building what? → novu kuću

In Croatian, a feminine singular noun used as a direct object usually goes into the accusative case.

• nominative: nova kuća = a new house
• accusative: novu kuću = a new house, as the object

The adjective must match the noun, so both words change:

• nova → novu
• kuća → kuću

Because kuća is a feminine singular noun, and here it is in the accusative case.

For many feminine nouns ending in -a:

• nominative singular ends in -a
• accusative singular ends in -u

So:

• kuća → kuću
• žena → ženu
• škola → školu

This is a very common pattern in Croatian.

Adjectives in Croatian must agree with the noun in:

• gender
• number
• case

Since kuću is:

• feminine
• singular
• accusative

the adjective must also be feminine singular accusative:

• nominative: nova kuća
• accusative: novu kuću

So novu matches kuću.

Because u can take different cases depending on meaning.

Here, u gradu means in the city, which expresses location, not movement.
For location, u usually takes the locative case.

• grad = city
• locative singular: gradu
• u gradu = in the city

If you were talking about movement into the city, you would normally use the accusative:

• u grad = into the city

So:

• grade novu kuću u gradu = they are building a new house in the city
• idu u grad = they are going into/to the city

Gradu is the locative singular of grad (city).

For many masculine nouns, the locative singular ends in -u:

• grad → u gradu
• stan → u stanu
• park → u parku

In this sentence, u gradu is a prepositional phrase showing location.

It can mean both.

Croatian present tense often covers both:

• they build
• they are building

Which one is meant depends on context.

So Oni grade novu kuću u gradu. could mean:

• They are building a new house in the city.
• They build a new house in the city.

In practice, the progressive-style meaning (are building) is often understood from context.

No, Croatian word order is fairly flexible, though some orders sound more neutral than others.

The most neutral version here is:

• Oni grade novu kuću u gradu.

But other orders are possible, depending on emphasis:

• Novu kuću grade u gradu.
• U gradu oni grade novu kuću.
• Novu kuću oni grade u gradu.

Changing the order usually changes focus or emphasis, not the basic meaning.

Yes. Croatian often relies on context, but you can add a time word if you want to be clearer.

Examples:

• Oni sada grade novu kuću u gradu. = They are building a new house in the city now.
• Oni često grade kuće u gradu. = They often build houses in the city.
• Oni ove godine grade novu kuću u gradu. = They are building a new house in the city this year.

Time expressions help clarify whether the action is happening now, regularly, or over a longer period.

The letter ć is a soft Croatian consonant. For English speakers, it is somewhat like a very soft ch/t sound, though there is no exact English equivalent.

So kuću sounds roughly like:

• KOO-chyu or KOO-tyu, approximately

A few pronunciation notes:

• ku = like koo
• ć = soft consonant
• u = like oo

Also:

• č and ć are different letters in Croatian
• learners often mix them up at first

Yes, if the subject is they referring to an all-female group.

Croatian distinguishes plural they by gender:

• oni = masculine mixed group, or a group with at least one male, or sometimes general they
• one = all-female group
• ona = neuter plural

So:

• Oni grade novu kuću u gradu. = they are building...
• One grade novu kuću u gradu. = they (all women) are building...

The verb form grade stays the same.

Yes, graditi is an imperfective verb.

That means it usually focuses on:

• the process
• the ongoing action
• repeated or habitual action

So Oni grade novu kuću u gradu. naturally suggests:

• they are in the process of building it
• or they build as an activity, depending on context

The related perfective verb is often izgraditi (to build, to complete building).

Compare:

• Oni grade kuću. = They are building a house / They build houses.
• Oni su izgradili kuću. = They built the house / They finished building the house.

This aspect difference is very important in Croatian.