Budući da je radio puno radno vreme, primio je platu.

Here budući da means since, because, or given that.

It does not express future time in this sentence, even though budući is related to the idea of future in other contexts. As a fixed conjunction, budući da simply introduces a reason:

• Budući da je radio... = Since/Because he worked...

Very common alternatives are:

• jer = because
• pošto = since / after / because (depending on context)

So here it is best understood as a conjunction of cause.

The comma separates the subordinate clause from the main clause.

• Budući da je radio puno radno vreme = dependent clause giving the reason
• primio je platu = main clause

In Serbian, when a clause introduced by something like budući da comes first, it is normally separated by a comma.

Serbian often drops subject pronouns when they are clear from context.

So instead of saying:

• On je radio...
• On je primio...

Serbian very naturally says just:

• je radio
• primio je

The sentence still means he worked / he received, because the verb form and context make the subject understandable. This is very common in Serbian.

Je radio is the perfect tense of raditi = to work.

The Serbian perfect is usually made with:

• the present tense of biti = to be
• plus the past active participle

So here:

• je = auxiliary, third person singular
• radio = past participle, masculine singular

Together:

• je radio = he worked / he was working / he has worked
  (the exact English translation depends on context)

If the subject were feminine, you would get:

• je radila = she worked

This is because je is a clitic in Serbian, and clitics usually go in the second position of their clause.

In the subordinate clause:

• da je radio
• after da, the clitic je appears early in the clause

In the main clause:

• primio je platu
• the first word is primio, so the clitic je comes right after it

This movement is normal in Serbian and is one of the things English speakers often need time to get used to.

They are both past participles used in the perfect tense, but they come from verbs of different aspect:

• radio comes from raditi = to work, an imperfective verb
• primio comes from primiti = to receive, a perfective verb

Why this combination?

• radio describes an ongoing or general activity/state: he was working full-time
• primio describes a completed event: he received a salary

This is a very typical Serbian aspect contrast:

• imperfective for background/process
• perfective for a completed result

Puno radno vreme means full-time work or full working hours.

More literally:

• puno = full
• radno = working / work-related
• vreme = time

As a whole, it functions like the English expression full-time.

So:

• radio puno radno vreme = worked full-time

A common contrast would be:

• puno radno vreme = full-time
• nepuno radno vreme or skraćeno radno vreme = part-time / reduced hours

Because platu is the accusative singular of plata.

• plata = nominative, the dictionary form
• platu = accusative, used for a direct object

Here, salary is what he received, so it is the direct object of primiti:

• primio je platu = he received a salary

This is a regular feminine noun pattern:

• žena → ženu
• knjiga → knjigu
• plata → platu

Yes, if you translate it word for word, it sounds like worked full working time, but that is just how Serbian expresses the idea.

It is a natural Serbian collocation. English uses full-time as an adverb/adjective much more compactly, while Serbian often uses the full phrase:

• raditi puno radno vreme = to work full-time

So even if the literal structure feels unusual from an English point of view, the phrase itself is normal and idiomatic.

Possibly, depending on context, but the Serbian sentence itself more directly says:

• he worked full-time
• he received a salary

It does not explicitly say he was employed. That would require a different wording, such as something based on biti zaposlen.

So the safest interpretation is the one closest to the Serbian wording:

• Since he worked full-time, he received a salary.

Yes. Some common alternatives are:

• Jer je radio puno radno vreme, primio je platu.
• Pošto je radio puno radno vreme, primio je platu.

But they are not all identical in style:

• budući da sounds a bit more formal or written
• jer is very common and straightforward
• pošto is also common, but in some contexts it can also mean after, so context matters

So budući da is perfectly correct here, but it has a slightly more formal feel than the simplest everyday jer.