Vidim ih u parku.

Because Serbian usually does not need the subject pronoun when the verb form already shows who is doing the action.

Here, vidim means I see, so ja is understood automatically.

• vidim = I see
• vidiš = you see
• vidi = he/she/it sees

You can add ja if you want emphasis, but it is not necessary:

• Ja ih vidim u parku. = I see them in the park.

Vidim is the 1st person singular present tense form of videti, meaning to see.

So:

• videti = to see
• vidim = I see

The ending -im is what tells you the subject is I.

Ih means them.

More specifically, it is the accusative plural clitic pronoun, used as the direct object of the verb.

In this sentence, the direct object is the people or things being seen, so Serbian uses ih.

A useful point: ih can refer to people or things, and it does not tell you the gender by itself. The context tells you who or what them refers to.

Because ih is a clitic. Clitics are short, unstressed words that usually cannot stand alone and usually appear in second position in the sentence.

In Vidim ih u parku, the first main element is vidim, so the clitic ih comes right after it.

This is natural word order:

• Vidim ih u parku.

You generally would not use:

• Ih vidim u parku.

unless you are doing something very marked or stylistic.

Because after u, Serbian uses different cases depending on the meaning:

• u + locative = location, in/at
• u + accusative = movement toward, into/to

Here the meaning is location: they are already there, and you see them there. So Serbian uses the locative:

• u parku = in the park

Compare:

• Vidim ih u parku. = I see them in the park.
• Idem u park. = I am going to the park.

Because Serbian nouns change form depending on their case.

The basic form is:

• park

But after u with location, you need the locative singular, so it becomes:

• u parku

This is a normal pattern for many masculine nouns.

Because Serbian does not have articles like English a and the.

So park can mean:

• a park
• the park

The exact meaning depends on context.

In u parku, English naturally translates it as in the park, but Serbian does not need a separate word for the.

Yes, Serbian word order is more flexible than English, but it is not completely free. The biggest restriction here is that ih is a clitic, so it must stay in a clitic position.

Neutral word order:

• Vidim ih u parku.

You can also say:

• U parku ih vidim.

This puts more emphasis on where you see them.

You can also say:

• Ja ih vidim u parku.

This emphasizes I.

But you cannot move ih just anywhere. For example, Vidim u parku ih sounds wrong in standard Serbian.

Most directly, u parku means in the park. But in real usage, Serbian u often covers situations where English might sometimes say at.

So depending on context, English may choose:

• in the park
• sometimes at the park

But the Serbian expression is still u parku.