Learn Italian: C'è uno sbaglio sullo schermo. - There is a mistake on the screen.

Sullo combines the preposition su (on) and the definite article lo (the). We must use lo here because schermo is a masculine noun starting with an 's' plus a consonant ('c'). If the word started with a regular consonant, we would use il, and the combined word would be sul.

Does uno sbaglio follow the same rule as sullo schermo?

Exactly! Both sbaglio and schermo are masculine words that start with an 's' followed by another consonant. Because of this tricky starting sound, they need special articles: uno for "a" (uno sbaglio) and lo for "the" (which turns su into sullo schermo).

Is it okay to write su lo schermo as two separate words?

No, in Italian, you must combine the preposition su with the definite article that follows it. Writing su lo separately is grammatically incorrect; they must always merge into the single word sullo.

What does the apostrophe in C'è stand for?

C'è means "there is." It is a contraction of ci (there) and è (is). The 'i' is dropped to make it flow better because the next word starts with a vowel.