Lepljiva traka i marker su na kutiji.

Because the subject is compound: lepljiva traka i marker = the sticky tape and the marker.

In Serbian, when two things are joined by i (and), the verb is usually plural, so you use su (are), not je (is).

• Lepljiva traka je na kutiji. = The sticky tape is on the box.
• Marker je na kutiji. = The marker is on the box.
• Lepljiva traka i marker su na kutiji. = The sticky tape and the marker are on the box.

Literally, lepljiva traka means sticky tape.

It is made of:

• lepljiva = sticky
• traka = tape / strip / ribbon

So the phrase works like an adjective + noun combination:

• lepljiva traka = sticky tape

The adjective lepljiva agrees with traka in gender, number, and case.

Because traka is a feminine singular noun in the nominative here, and the adjective has to match it.

So:

• traka = feminine singular
• therefore lepljiva = feminine singular form of lepljiv (sticky)

Compare:

• lepljiva traka = sticky tape (feminine singular)
• lepljivi marker = sticky marker (masculine singular)
• lepljivo nešto = something sticky (neuter singular)

This matching is called agreement.

Because the preposition na here means on, and for location it requires the locative case.

The base form is:

• kutija = box

But after na when you mean on the box, it changes to:

• na kutiji

So:

• kutija = box
• na kutiji = on the box

This is one of the most common case patterns in Serbian:

• na stolu = on the table
• na polici = on the shelf
• na kutiji = on the box

It is locative singular.

In this sentence, na kutiji answers the question Where?, so Serbian uses the locative after na.

A useful distinction is:

• na + locative = location, on
• na + accusative = movement toward a surface, often onto

For example:

• Marker je na kutiji. = The marker is on the box.
• Stavljam marker na kutiju. = I am putting the marker onto the box.

So in your sentence, nothing is moving; it is just describing location.

Because Serbian does not have articles like English a/an/the.

So a sentence like:

• Lepljiva traka i marker su na kutiji

can mean:

• The sticky tape and the marker are on the box
• or sometimes A sticky tape and a marker are on a box

Which meaning is intended depends on context.

English learners often want to insert the, but Serbian simply does not use articles in the same way.

Yes. Marker is used in Serbian and is a loanword.

Depending on context, speakers may also use other words, such as:

• flomaster = felt-tip pen / marker
• marker = marker

The exact nuance can vary a bit depending on region and speaker, but marker is completely understandable and natural in many contexts.

Yes. Serbian word order is more flexible than English word order.

The neutral order here is:

• Lepljiva traka i marker su na kutiji.

But you may also hear:

• Na kutiji su lepljiva traka i marker.

This version puts more focus on where they are.

Because Serbian uses cases, word endings help show the grammatical relationships, so word order can move around more freely than in English. Still, not all orders sound equally natural in every context.

A rough pronunciation is koo-TEE-yee.

Breaking it down:

• ku = like koo
• ti = tee
• ji = a yee-like sound at the end

So:

• kutiji ≈ koo-TEE-yee

Also:

• j in Serbian is pronounced like English y in yes
• so ji sounds like yi

Because Serbian normally uses i once to join two nouns, just like English and:

• lepljiva traka i marker = sticky tape and marker

You do not need to repeat it. Repeating conjunctions is possible in some special stylistic or emphatic situations, but the normal simple structure is exactly what you see here.

Not in standard Serbian. Box is not the normal Serbian word here.

You should use:

• kutija = box
• na kutiji = on the box

If you are learning, it is best to memorize the whole pattern:

• kutija
• u kutiji = in the box
• na kutiji = on the box
• na kutiju = onto the box

That will help you build correct case forms naturally.