"She studies every afternoon."
Translation:Ella estudia todas las tardes.
That's just one way to say every in Spanish. I guess you could think of it as saying all the afternoons if that helps. But it's not translated that way, but rather every afternoon.
Every day = todos los días
How do I know when to use "todos" vs "todas". I know one is fem. and the other is mas.
You have to see which gender the noun has that the adjective is describing. In this case it's tarde, a feminine noun, so if you want to say "every evening", you need the plural feminine version: "todas las tardes".