Papir i olovka su na stolu.

Because the subject is plural: two nouns joined by i form a compound (plural) subject, so you use the 3rd person plural of biti (to be), which is su. With a single subject you’d use je:

• Papir je na stolu.
• Olovka je na stolu.
• Papir i olovka su na stolu.

After na, Croatian uses:

• Locative for static location: na stolu = on the table.
• Accusative for movement onto: Stavljam papir i olovku na stol = I’m putting the paper and the pencil onto the table.

Note: For inanimate masculine nouns like stol, the accusative equals the nominative (both are stol), which is why you don’t see a change there, but you do in the locative (stolu).

Croatian has no articles. Definiteness is inferred from context. If you need to be explicit, use demonstratives:

• taj papir i ta olovka = that paper and that pencil
• onaj papir i ona olovka = that paper and that pencil (farther away)

Yes. Both orders are correct:

• Papir i olovka su na stolu.
• Na stolu su papir i olovka.

Starting with Na stolu emphasizes the location a bit more. Notice that su stays in second position in the clause.

Subjects are in the nominative case: papir, olovka. Forms like olovku (accusative) or papira (genitive) are used for other functions:

• Subject: Olovka je na stolu.
• Direct object: Vidim olovku.
• Possession/amount: List papira (a sheet of paper).

• papir is masculine
• olovka is feminine

In the present tense, the verb (su) doesn’t show gender. But adjectives and past participles agree in plural. With mixed genders, masculine plural is used:

• Past: Papir i olovka su bili na stolu. If both were feminine, you’d use feminine plural:
• Olovka i knjiga su bile na stolu. If you replace them with a pronoun, mixed gender takes masculine plural: Oni su na stolu.

U stolu would literally mean “inside the table,” which is odd unless you mean a drawer. More natural is:

• Olovka je u ladici (stola). = The pencil is in the drawer (of the table). Use na for surfaces (na stolu) and u for insides (u ladici).

Use nisu (negative 3rd person plural of biti):

• Papir i olovka nisu na stolu.

Use the full verb form with li:

• Jesu li papir i olovka na stolu? Short answers use the full forms:
• Jesu. / Nisu. You can also ask a wh‑question:
• Gdje su papir i olovka? = Where are the paper and the pencil?

All of the above, depending on context:

• Material (mass): Treba mi papir. = I need paper.
• A sheet: list papira (literally “sheet of paper”).
• Documents: papiri = papers/documents.

If you mean two sheets, say dva lista papira. Dva papira is usually understood as “two documents.”