Learn Italian: Leggo un libro sulla poltrona. - I read a book on the armchair.

Sulla is a combination of the preposition su (on) and the definite article la (the). Because poltrona (armchair) is a feminine singular noun, it takes the article la. When you combine su and la, you get sulla. You would use sul for masculine words that use the article il.

Why does sulla have two Ls when the article la only has one?

When you combine the preposition su with a definite article that starts with L (like la, lo, le, or l'), Italian spelling rules require you to double the L. That is why su plus la becomes sulla.

In English we say we sit 'in' an armchair. Why does Italian use su (on)?

Prepositions rarely translate perfectly word-for-word between languages. While English speakers often talk about sitting deep 'in' an armchair, Italians talk about sitting 'on' it. Therefore, you use su (combined here as sulla) for reading or sitting on chairs, armchairs, and couches.

Why doesn't the sentence start with io (I)?