Ovaj lek je stvarno dobar.

Because lek is a masculine singular noun, and the demonstrative ovaj must agree with it in gender, number, and case.

• ovaj = masculine singular
• ova = feminine singular
• ovo = neuter singular

So with lek, you say ovaj lek.

It is in the nominative singular.

That is because ovaj lek is the subject of the sentence. In Serbian, the subject is normally in the nominative case.

• ovaj = nominative masculine singular
• lek = nominative singular

Je is the 3rd person singular present tense of the verb biti (to be).

So in this sentence, it functions like is in English. Serbian often uses forms of biti in exactly this kind of sentence:

• On je visok. = He is tall.
• Ovaj lek je dobar. = This medicine is good.

Without je, the sentence would sound incomplete in standard Serbian.

Because adjectives in Serbian must agree with the noun they describe.

Since lek is:

• masculine
• singular
• nominative

the adjective must also be:

• masculine
• singular
• nominative

So you get dobar.

Compare:

• dobar lek = masculine
• dobra knjiga = feminine
• dobro vino = neuter

Here stvarno is an adverb meaning something like really, truly, or genuinely.

It strengthens the statement:

• Ovaj lek je dobar. = neutral statement
• Ovaj lek je stvarno dobar. = stronger, more emphatic

In everyday speech, stvarno is very common and natural.

Yes, word order in Serbian is fairly flexible, though some versions sound more natural than others.

Common possibilities include:

• Ovaj lek je stvarno dobar.
• Ovaj lek je dobar stvarno. — possible, but less neutral
• Stvarno je ovaj lek dobar. — more marked/emphatic

The most natural basic version is the one you have: Ovaj lek je stvarno dobar.

Lek usually means medicine, medication, or remedy. In some contexts, English might translate it as drug, but learners should be careful because English drug can also suggest narcotics, while lek normally does not.

So lek is the ordinary word for something used to treat illness.

Sometimes yes, but it is not exactly the same in usage.

• lek is the everyday general word for a medicine/medication
• medicina exists too, but it often refers more broadly to medicine as a field, or in some contexts a medicinal preparation

For a specific pill, syrup, or treatment, lek is usually the most natural choice.

Because Serbian does not need a dummy subject like English it in this kind of sentence.

In English you might say:

• This medicine is really good.

In Serbian, the subject is already ovaj lek, so nothing extra is needed.

Also, Serbian often leaves out personal pronouns when they are not necessary, because the verb form already gives useful information.

A simple learner-friendly pronunciation guide would be:

OH-vai lek ye STVAHR-no DOH-bar

A few helpful notes:

• j is pronounced like English y
• je sounds roughly like ye
• r in Serbian is a trilled or tapped r
• stress can vary by dialect and accent, but this approximation is enough for a beginner

It is used for both. Dobar is a very common adjective meaning good, and it can describe:

• people: dobar čovek = a good person
• objects: dobar telefon = a good phone
• food: dobra kafa = good coffee
• medicine: dobar lek = good/effective medicine

With lek, it often implies that the medicine is effective or helpful.

Yes. That would still be grammatical.

The difference is:

• Ovaj lek... points to a specific medicine: this medicine
• Lek... can mean the medicine or medicine depending on context

So if you are talking about one specific medicine in front of you or already mentioned, ovaj lek is more precise.

It is neutral and natural in both everyday speech and normal writing.

Nothing in Ovaj lek je stvarno dobar is slangy or overly formal. The word stvarno is common in spoken language, but it is still perfectly normal in ordinary written Serbian too.

If you wanted a slightly more formal tone, you might sometimes use zaista instead of stvarno:

• Ovaj lek je zaista dobar.

Both are correct.