why and how did helen learn french

From the text:

I had a French grammar in raised print, and as I already knew some French, I often amused myself by composing in my head short exercises, using the new words as I came across them, and ignoring rules and other technicalities as much as possible. I even tried, without aid, to master the French pronunciation, as I found all the letters and sounds described in the book. Of course this was tasking slender powers for great ends; but it gave me something to do on a rainy day, and I acquired a sufficient knowledge of French to read with pleasure La Fontaine's "Fables," "Le Medecin Malgre Lui" and passages from "Athalie."