Learn French: Tu dois lire ce livre australien. - You must read this Australian book.

To make adjectives ending in -en feminine, you double the final n and add an e. So, if you were talking about an Australian song (une chanson), the adjective would become australienne.

Why is lire in its dictionary form instead of being conjugated?

When you have two verbs back-to-back, the first one is conjugated (dois) and the second one stays in its infinitive or dictionary form (lire). This is a very common pattern in French.

Since livre ends in an e, shouldn't we use the feminine cette instead of ce?

It is tricky, but livre (book) is actually a masculine noun despite ending in an e. Because it is masculine, we use the masculine word for this, which is ce.

Which verb is dois from?

The word dois comes from the verb devoir, which means must or to have to. It is an irregular verb, meaning it doesn't follow standard ending patterns and its forms like dois just need to be memorized.