Imagine that in Spanish instead of using the word "otro" you had to use the word "diferente". Would you use "un" / "una"? Then it's probably "another" in English. Relying a bit on Google Translate: Ayer vi a otro amigo. Ayer vi a un amigo diferente = I saw a different friend yesterday Therefore: I saw another friend yesterday. El mago puede usar magia de fuego y también otra magia. El mago puede usar magia de fuego y magia diferente también = The wizard can use fire magic, and different magic too Therefore: The wizard can use fire magic, and other magic too. In other words, it is a complexity of Spanish and Portuguese grammar which often makes you drop the indefinite article with the word for "other", whereas with a different adjective you would include the indefinite article. "An other" normally becomes "another" (possible exceptions include when "other" is used with special emphasis as a noun). "Other" can also be used in the phrase "other than" (to mean "aparte de").

"other" = "an" + "other" The most important distinction is that "other" requires a plural whereas "another" requires a singular. "I want another book" means I want a (single) book in addition to whatever books I already have. "I want other books" means I want books besides ... (besides what? it depends on the rest of the conversation.) The words can also be used as pronouns. For example, in pop music you often hear "I will never love another" but you will never hear "I will never love others" although it makes sense. The first is better because it clearly states that there is not one other person whom you will ever love. The second means the same, but we don't say it that way. Notice how you need to add the "s" to "other" when you use it this way.