Radim deset ponavljanja.

Radim is the 1st person singular present tense of raditi.

So radim means I do or I am doing, depending on context.

In this sentence, it naturally means something like:

• I’m doing ten repetitions
• or I do ten repetitions

Serbian present tense often covers both the simple present and the present continuous that English separates.

Because Serbian usually doesn’t need a subject pronoun when the verb already shows who is doing the action.

Radim already means I do / I am doing, so ja is unnecessary unless you want emphasis.

• Radim deset ponavljanja. = neutral, normal
• Ja radim deset ponavljanja. = I’m doing ten repetitions (maybe contrasting with someone else)

Yes.

That is very common in Serbian. The present tense of an imperfective verb like raditi can express:

• a current action: I’m doing ten repetitions
• a regular habit: I do ten repetitions

Context tells you which meaning is intended.

For example:

• at the gym, during the set: Radim deset ponavljanja.
• talking about your routine: Obično radim deset ponavljanja. = I usually do ten repetitions.

Because after numbers like pet, šest, sedam... deset and higher, Serbian normally uses the genitive plural of the noun.

The noun here is ponavljanje = repetition.

After deset, it becomes:

• deset ponavljanja

So the pattern is:

• jedno ponavljanje = one repetition
• deset ponavljanja = ten repetitions

Here it is functioning as genitive plural after the number deset.

A detail that often confuses learners: with this noun, the form ponavljanja looks the same in more than one grammatical role. It can look like:

• nominative plural
• genitive plural
• and in some contexts other related forms

So in deset ponavljanja, it is plural in meaning: ten repetitions.

Yes, it is natural.

In workout or exercise contexts, Serbian commonly uses raditi for performing exercises, sets, or repetitions.

So:

• Radim deset ponavljanja. = I’m doing ten reps.

You can also hear more specific versions like:

• Radim deset ponavljanja sklekova. = I’m doing ten push-up reps.
• Radim tri serije po deset ponavljanja. = I’m doing three sets of ten reps.

So raditi works very naturally here.

This is mainly about aspect.

• radim is imperfective
• uradim is perfective

Radim deset ponavljanja focuses on the action as ongoing, habitual, or in progress:

• I’m doing ten repetitions
• I do ten repetitions

Uradim deset ponavljanja focuses on completing the action:

• I do/complete ten repetitions

But there is an important catch: in Serbian, a perfective present like uradim usually does not mean I am doing right now. It is more natural in contexts like:

• Mogu da uradim deset ponavljanja. = I can do ten repetitions.
• Uradio sam deset ponavljanja. = I did / completed ten repetitions.
• Uradiću deset ponavljanja. = I’ll do ten repetitions.

So for I’m doing ten reps, radim is the right choice.

Yes, Serbian word order is fairly flexible, but the neutral order here is:

Radim deset ponavljanja.

Other orders are possible, but they add emphasis or sound more marked:

• Deset ponavljanja radim. = emphasizes ten repetitions
• Ponavljanja radim deset. = possible, but less natural in most everyday contexts

For a learner, the safest and most natural version is: Radim deset ponavljanja.

Yes, but the meaning is a little different.

• deset ponavljanja = ten repetitions / ten reps
• deset puta = ten times

Deset puta is more general. It just says something happens ten times.

So:

• Radim deset ponavljanja. sounds especially natural in exercise/training language.
• Radim to deset puta. = I do that ten times.

If you are specifically talking about reps in a set, ponavljanja is better.

A helpful rough breakdown is:

po-nav-lja-nja

Two sounds are especially important:

• lj = a soft ly-like sound
• nj = a soft ny-like sound

So ponavljanja sounds roughly like: po-nav-LYA-nya

Not a perfect English spelling, but close enough to help.

Also:

• Radim = RA-dim
• deset = DE-set
• ponavljanja = po-nav-LYA-nya

The dictionary form is ponavljanje.

That is the nominative singular form, and it means repetition.

So the sentence is built from:

• raditi → radim
• deset
• ponavljanje → ponavljanja

When learning Serbian, it is useful to identify the dictionary form first, because the form you see in a sentence is often changed by case and number.