Learn Italian: Il libro è sul tavolo. - The book is on the table.

It is a combination of the preposition su (on) and the definite article il (the). In Italian, when a preposition like su is followed by a definite article, they must merge together into a single word, which is called an articulated preposition.

Why do we use sul here instead of sulla or sullo?

The combined word always matches the gender and number of the noun it points to. tavolo (table) is a masculine singular noun that starts with a regular consonant, so it takes the article il. Combining su + il gives us sul. If the book were on a chair (la sedia), you would use su + la to make sulla.

Can I just write su il tavolo as two separate words?

No, merging the preposition and the article is mandatory in Italian. You cannot leave them as separate words. Whenever su is directly followed by a definite article, you have to form the combined word.

I often forget the accent on è. Does it really matter?

Yes, it is essential! The accent mark changes the meaning of the word completely. è with the accent means "is" (from the verb essere, to be). e without the accent means "and". Without it, your sentence would translate to "The book and on the table."