Learn Italian: Il cane dorme sul cuscino. - The dog sleeps on the cushion.

sul is a combination of the preposition su (on) and the definite article il (the). Because cuscino (cushion) is a masculine noun starting with a consonant, it uses il. When you put them together, su and il become sul.

If the dog was sleeping on a chair (sedia) instead of a cushion, how would the word sul change?

sedia is a feminine noun, so it takes the article la (the). When you combine su (on) with la, it becomes sulla. The sentence would then be Il cane dorme sulla sedia.

What is the dictionary form of the verb dorme?

The infinitive is dormire (to sleep). Since Il cane (the dog) acts as a third-person singular subject just like he, she, or it, we drop the ire ending and add e to get dorme.

Why does the letter combination sc in cuscino sound like an English sh?