"The other one does not work."

Translation: La alia ne funkcias.

[tvoiles]

Why is it "alia" and not "alio"?

[mizinamo]

It's an adjective.

Like "La blua ne funkcias" (the blue one doesn't work).

English has to use adjective + "one" but Esperanto just uses the adjective in this cases.

"La bluo" would be "the concept of blue, blueness" or something like that, so I suppose the natural reading of "alio" would be "differentness".

That said, PIV says that "alio" can be used to mean "something else", but its examples do not have the definite article (i.e. "something else" rather than "the other thing). I don't think I've ever heard the word.

[donaldo_zouras]

When I read this sentence in English I though it was talking about another person that does not do their work.
La alia unu ne laboras.