Learn Italian: La panchina è sull'erba. - The bench is on the grass.

It is the combination of the preposition su (on) and the definite article l' (the). Because erba (grass) starts with a vowel, it takes the article l'. When you combine su and l', you get sull'.

Why does sull' have a double L?

When the preposition su combines with any definite article that starts with an L (like il, la, lo, or l'), the L doubles. That is why su + l' becomes sull'.

Could I just write sulla erba instead of using the apostrophe?

In standard Italian, it is highly preferred to drop the vowel and use an apostrophe here. When an articulated preposition like sulla comes right before a noun starting with a vowel, like erba, it becomes sull' to make pronunciation smoother.

What kind of bench is a panchina?

A panchina specifically refers to a public bench, like one you would sit on in a park or while waiting for a bus. For a regular indoor chair, you would use the word sedia.