'Para' is 'for' in this sentence. ...para aprender inglés = for learning English This tells you the reason why you are doing something. When you're talking about a book, or a course, or a lecture, or an article, for example, the preposition 'on' means 'on the subject of'. For example, if you are interested in archaeology, you'd say 'I want to borrow a book on archaeology.' I hope that helps.