Ovde mogu da se opustim.

Serbian often leaves out subject pronouns when they are already clear from the verb form.

Here, mogu means I can, so ja is understood automatically.
You could say Ja ovde mogu da se opustim, but that adds emphasis, like I can relax here.

Ovde means here.

Yes, ovde and ovdje mean the same thing. The difference is mainly regional/standard variety:

• ovde is the Ekavian form, common in Serbia
• ovdje is the Ijekavian form, common in Bosnia, Montenegro, and also in Croatian

So the meaning is the same; only the form changes.

Mogu is the 1st person singular present tense of moći, which means to be able / can.

So:

• ja mogu = I can
• ti možeš = you can
• on/ona može = he/she can

In this sentence, mogu tells you the subject is I, even though ja is not stated.

After modal verbs like moći, Serbian very often uses da + present tense instead of an infinitive.

So:

• mogu da se opustim = I can relax

This is one of the most common Serbian patterns. English learners often expect something more like can to relax, but Serbian does not work that way.

An infinitive version can exist in Serbian in some contexts, but da + present is the most natural everyday pattern here.

Because the verb is opustiti se or opuštati se, meaning to relax.

The se is part of the verb. It is a reflexive particle, but in many verbs like this, it is best learned together with the verb itself.

So do not think of the base verb here as just opustiti. Learn:

• opustiti se = to relax, to unwind
• opuštati se = to be relaxing, to relax habitually/repeatedly

Without se, the meaning changes.

This is a question of aspect.

Serbian has two aspectual partners here:

• opuštati se = imperfective
• opustiti se = perfective

In the sentence mogu da se opustim, the form opustim comes from the perfective verb opustiti se.

That suggests reaching the result: I can relax / I can manage to unwind here.

If you said mogu da se opuštam, that would sound more like:

• I can be relaxing here
• I can relax here regularly
• I have the opportunity to spend time relaxing here

So opustim focuses more on successfully getting into a relaxed state.

Yes. Grammatically, opustim is a present-tense form of the perfective verb opustiti se.

That can feel strange to English speakers, because perfective present forms in Serbian often do not behave like ordinary present tense in English.

But after a modal verb like mogu, this is completely normal.
Mogu da se opustim means I can relax, not necessarily future-only.

So the important point is:

• opustim is present in form
• after mogu, it expresses ability or possibility

Yes, Serbian word order is flexible.

The sentence Ovde mogu da se opustim is a very natural neutral version. But other orders are also possible, depending on emphasis:

• Mogu da se opustim ovde = emphasis on here
• Ja ovde mogu da se opustim = emphasis on I
• Ovde se mogu opustiti = another possible structure, a bit different in style

Even though the order can move around, not every position sounds equally natural in every context. The original sentence is a very good standard everyday version.

It can mean either, depending on context.

Most often, Ovde mogu da se opustim means something like:

• Here I can relax
• This is a place where I’m able to unwind
• I feel I can relax here

So it can describe:

• a present situation
• a general feeling
• a repeated experience

Serbian present forms often rely on context more than English does.

No. Ovde is an adverb, not a noun, so it does not have case here.

That is why there is no preposition and no noun ending to analyze. It simply means here.

English speakers sometimes look for a case ending everywhere in Serbian, but some words, like ovde, are just adverbs.