Learn French: Elle est fatiguée, et elles sont aussi fatiguées.

Elle means "she", and elles means "they" (for a group of females). They sound exactly the same! To know which one is being used in spoken French, you have to listen to the verb. Elle est (She is) uses the singular verb, while elles sont (they are) uses the plural verb.

Why do we use sont in the second half of the sentence?

Sont is the plural form of the verb être (to be) for "they". Just like in English you switch from "is" to "are", in French you switch from the singular est (is) to the plural sont (are) when talking about more than one person.

Why does the second fatiguées have an extra "s"?

In French, describing words (adjectives) have to match the number of people they describe. Because elles refers to a plural group, you must add an s to the end of fatiguée to show there is more than one person.

What is the extra "e" for in both fatiguée and fatiguées?