Piknik je u parku.

• Piknik = picnic. It is a noun.
• je = is. It is the 3rd person singular present form of biti (to be).
• u = in / at, depending on context.
• parku = park in the locative singular case, used after u when talking about location.

So grammatically, the sentence is literally Picnic is in park, which English expresses as The picnic is in the park.

Because after u meaning in/at a location, Serbian normally uses the locative case.

The dictionary form is park, but in the locative singular it becomes parku:

• park = park
• u parku = in the park

So u park would be incorrect for location.

Parku is in the locative singular.

For many masculine nouns, the locative singular often ends in -u:

• grad → u gradu = in the city
• park → u parku = in the park

So if you see u plus a place noun ending in -u, it is often the locative.

Serbian does not have articles like English a and the.

So:

• piknik can mean a picnic or the picnic
• u parku can mean in a park or in the park

The exact meaning depends on context. In your sentence, English usually translates it naturally as The picnic is in the park.

Yes. Piknik is a masculine noun.

A common clue is that many masculine nouns in Serbian end in a consonant in the basic form:

• piknik
• park
• grad

Because piknik is singular masculine, the verb is je (is).

Je is the present-tense form of biti (to be) for he/she/it.

Since piknik is singular, Serbian uses:

• Piknik je... = The picnic is...

Compare:

• Piknik je u parku. = The picnic is in the park.
• Piknici su u parku. = The picnics are in the park.

So je agrees with the singular subject.

In Serbian, short forms like je are often clitics, which tend to appear in the second position of the sentence.

So:

• Piknik je u parku. is the normal order.

You may also hear different word orders for emphasis, such as:

• U parku je piknik. = It is in the park that the picnic is / The picnic is in the park.

But in your sentence, je after the first word is completely standard.

Yes. Serbian word order is more flexible than English, although the meaning stays similar.

Examples:

• Piknik je u parku. = neutral statement
• U parku je piknik. = emphasis on the location
• U parku piknik je = possible in poetry or very marked speech, but not the normal everyday order

So the original sentence is the most neutral and natural version for a learner.

Not always. U can mean in, inside, or sometimes at, depending on context.

Examples:

• u parku = in the park
• u školi = at school / in school
• u Beogradu = in Belgrade

So in this sentence, u is best translated as in.

A simple English-friendly approximation is:

PEEK-neek yeh oo PAR-koo

More roughly:

• Piknik = peek-neek
• je = yeh
• u = oo
• parku = par-koo

A few helpful pronunciation notes:

• j in Serbian sounds like English y in yes
• u is like oo in food
• Serbian pronunciation is usually quite phonetic, so words are generally pronounced close to how they are written

In standard Serbian, no—you normally keep je in a sentence like this.

So:

• Piknik je u parku. = correct
• Piknik u parku. = not a normal complete sentence in standard Serbian

Serbian usually requires the verb to be in present-tense location statements like this.

You would change both the noun and the verb:

• Piknik je u parku. = The picnic is in the park.
• Piknici su u parku. = The picnics are in the park.

Changes:

• piknik → piknici = picnics
• je → su = are

The phrase u parku stays the same here because it is still singular: in the park.