Learn Swedish: Hon har trettio böcker.

Bok is an irregular noun in Swedish. When it becomes plural, the root vowel changes from o to ö, and it takes the ending -er. You will see this vowel change in a few other common words, just like "foot" changes to "feet" in English.

How is the number trettio usually spoken?

Although it is spelled trettio, native speakers almost always drop the final -o in everyday speech and pronounce it as tretti. This happens with most Swedish numbers ending in -tio.

If the subject was "I" or "they" instead of "she" (hon), would the verb har change?

No, and that is one of the best things about Swedish verbs! A verb stays exactly the same in the present tense regardless of who is doing the action. It is jag har (I have), hon har (she has), and de har (they have).

Does the word hon sound like the English word "hon"?

No, the pronunciation is quite different. The vowel o in hon sounds more like the short "oo" sound in the English word "book" or "foot".