Learn French: Elle est assez fatiguée.

We use phrases with de (like assez de or beaucoup de) when we are talking about a quantity of a noun (a thing), like assez de temps (enough time). But here, assez is modifying an adjective (fatiguée). When these words modify adjectives instead of nouns, you drop the de.

I thought assez meant 'enough'. Why is it translated as 'quite' here?

Assez is a very flexible word! While it means 'enough' when talking about quantities, when you put it right before an adjective (like fatiguée), it generally translates to 'quite' or 'rather'. To a French speaker, being 'enough tired' is basically the same as being 'quite tired'.

Why does fatiguée end with a double 'e' (-ée)?

Because it describes a feminine subject, Elle (she). In French, adjectives must agree with the gender of the person or thing they describe. The masculine spelling is fatigué, and we add an extra e to make it feminine (fatiguée). Don't worry about pronouncing it differently, though—both forms sound exactly the same!