Learn Italian: Il gatto dorme su un letto comodo. - The cat sleeps on a comfortable bed.

Because the word after it is un, which is an indefinite article (a/an). In Italian, prepositions only combine with definite articles (words for "the", like il or la). If the sentence were "on the bed," it would become sul letto (su + il). Since it is "on a bed," su and un stay as two separate words.

Does the preposition su always mean "on"?

Usually, yes! Su translates to "on" or "onto" in most cases, especially when talking about physical location like resting on a bed. You will also see it used to mean "about" when talking about the topic of a conversation, a book, or a movie.

Why does the adjective comodo come after letto instead of before it?

In Italian, most descriptive adjectives go after the noun they describe. So, instead of saying "a comfortable bed" like we do in English, you say "a bed comfortable" (un letto comodo).

Why does the verb dorme end in an -e?