Kandidat je radio u klubu.

A word-for-word breakdown is:

• kandidat = candidate
• je = is/has as an auxiliary verb here
• radio = worked (masculine singular past participle of raditi)
• u = in / at
• klubu = club in the locative singular

So the structure is literally something like Candidate has worked in/at club, but in natural English it is The candidate worked in a club or The candidate was working at the club, depending on context.

Je is the 3rd person singular present form of biti (to be), and it is used as an auxiliary to form the past tense.

So:

• radi = works / is working
• je radio = worked / was working / has worked depending on context

Croatian commonly forms the past tense with:

• a form of biti
• plus a past participle

Here that gives:

• je radio = worked

Yes, radio can also be a noun in Croatian, just like in English. But in this sentence it is clearly the verb form worked.

You can tell because of the structure:

• je radio = past tense verb phrase

Here radio is the masculine singular past participle of raditi (to work).

So in this sentence:

• radio = worked
• not the noun radio

It is pronounced in three syllables: ra-di-o.

Because u changes case depending on meaning.

With location (in / at), u takes the locative:

• u klubu = in the club / at the club

With motion toward/into, u takes the accusative:

• u klub = into the club

So:

• Radio je u klubu = He worked in/at the club
• Ušao je u klub = He entered the club / into the club

Since working happens in a place, not into a place, klubu is correct.

Klubu is locative singular of klub.

The basic form is:

• klub = nominative singular

After u when it means in/at, you use the locative:

• u klubu = in/at the club

This is a very common pattern in Croatian:

• u gradu = in the city
• u školi = at school / in the school
• u kući = in the house / at home

It can mean either, depending on context.

Croatian u often covers both English in and at when talking about location.

So u klubu could be understood as:

• in the club
• at the club

In many real contexts, English would prefer at the club if you mean the workplace or venue in general.

Croatian does not have articles like English the and a/an.

So kandidat can mean:

• a candidate
• the candidate

And u klubu can mean:

• in a club
• in the club
• at a club
• at the club

Context tells you which meaning is intended.

That is why one Croatian sentence can match several natural English translations.

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

The neutral order here is:

• Kandidat je radio u klubu.

But other orders are possible for emphasis, for example:

• U klubu je radio kandidat.
• Kandidat je u klubu radio.

These do not usually change the basic meaning, but they can shift focus or sound more marked.

One important point: je is a clitic, so it usually comes in the second position of the clause or after the first stressed unit.

That is why je appears where it does.

It can translate in different ways depending on context.

The verb raditi is imperfective, so je radio can mean:

• worked
• was working
• used to work

Examples:

• Kandidat je radio u klubu prošle godine. = The candidate worked / used to work at a club last year.
• Kad sam ga vidio, kandidat je radio u klubu. = When I saw him, the candidate was working in the club.

So the Croatian form stays the same, while English chooses the most natural tense from context.

Because the past participle agrees with the subject in gender and number.

Here kandidat is masculine singular, so you get:

• radio = masculine singular

Other forms would be:

• radila = feminine singular
• radilo = neuter singular
• radili = masculine personal plural or mixed-gender plural
• radile = feminine plural

So for example:

• Kandidatkinja je radila u klubu. = The female candidate worked in a club.
• Kandidati su radili u klubu. = The candidates worked in a club.

Not always. Kandidat can mean different things depending on context, such as:

• candidate
• applicant
• nominee

In this sentence, the exact English choice depends on the wider situation. Grammatically, though, it is simply the subject of the sentence in the nominative singular.