As a native speaker of English, I can safely say you never stop learning new words and new rules.  English is an incredibly complex language, and with the popularity of the internet, the language (as well as other languages as well) continue to evolve.  I honestly think one will never be "done" learning English.  There are many different things to learn as the basics, but like anyone learning a second language, time exposed and how hard you work will have a lot to do with how easily you learn it.  Japanese and English are VERY different languages, and as someone who is learning Japanese as their second language, learning the basics has been an interesting ride.