"You read a book."
Translation:Tú lees un libro.
Umm yeah this is a difficult concept for English speakers. Nouns in Spanish all are either masculine and feminine, and you kinda just have to remember it. The main reason why you need to remember it is so that you know whether to say "el/la" in front of it and to make adjective agree with it. Here's a link that might help! http://www.spanishdict.com/topics/show/1
Nope, "un/una" can be either "a" or "an". You choose the correct indefinite article by looking at the noun that follows it and matching the noun in number and gender. These links have more information about this: http://www.spanishdict.com/topics/show/6 http://www.studyspanish.com/lessons/defart1.htm
Actually, "leias" was not preferred, when DL decided your answer was wrong it picked another "correct" answer to show you. Even though you are not working with vosotros endings it decided it was a good idea to show you that as a possible correct answer. It dinged me for using the usted form that for some reason has been left off the list of correct responses. Considering the size of the database with which they work Viva el DL!