1. Before numbers following names of sovereigns, etc., as—
Carlos I (primero), Charles I (the first)
Alfonso XIII (trece), Alphonso XIII (the thirteenth)
2. In titles of books, headings, etc., as—Historia de la Inquisición (The History of the Inquisition).
3. Before words in apposition, as—Madrid, capital de España (Madrid, the capital of Spain).
But—
Alfonso el Sabio (Alphonso the Wise) Juana la Loca (Jane the Mad) because these are "titles."
The =Indefinite Article= is used in English and not in Spanish (besides rules in Lesson II)—
1. Before words in apposition, as— Rubio y Cía., casa importantísima de la Habana: Rubio & Co., a most important firm in Havana.
2. In titles of books, headings, etc., as— Lista de los géneros pedidos: A list of goods required.
3. In "such a," "so … a," as— Tal amigo: Such a friend. Tan buen amigo: So good a friend, such a good friend.