Learn Italian: I bambini giocano su un tappeto.

Su combines with definite articles (the words for "the", like il or la) to make combined words like sul or sulla. But here, the article is un (an indefinite article, meaning "a"). Prepositions in Italian never combine with indefinite articles, so they just stay as two separate words: su un.

Does su always mean "on"?

Usually, yes! It is the standard preposition for a physical location on top of something (like on a rug, on a table). Later on, you will also see it used to mean "about" a topic (like a book about history) or "up" in directions.

I always mix up the plural words for "the". Why is it i bambini and not gli bambini?

The singular word is il bambino because it starts with a normal consonant (b). The plural of the article il is always i. You only use gli for plural masculine nouns that start with a vowel or "special" consonants (like z, ps, gn, or s + another consonant).

How do I pronounce giocano? Where does the emphasis go?

The stress is on the first part of the word! It is pronounced GIO-ca-no. In Italian, most regular verbs in the "they" form (third-person plural) have the stress on the third-to-last syllable, which often catches beginners off guard.