In Katy Perry's song, "By the Grace of God", in the first sentence she sings
Was 27 surviving my return to Saturn
Google shows an album from a Japanese band, which AFAIK Katy has nothing to do with. The background is that her ex-husband said he wanted a divorce, and the song was written...
Still in You still have much to learn added something to the sentence. It suggests that you've learned some, and you probably think you've learned a lot, and probably have mastered it, but no, you still have much to learn.
@CowperKettle Personally, I don't think there's anything wrong with rapidly developing. It's like a developing branch or field of something. Quickly developing, rapidly developing, either should work.
@yubraj I don't know if there's a book for that. I guess there is. Something like a prep book for non-native teachers. But I don't know. I don't think we really need that kind of book. Or more precisely, we shouldn't strictly follow that kind of book. It makes our discourses or instructions mechanical and lifeless, IMHO.
@AraucariaMan I understand that you want to discuss this point ("Not making mistakes is no indication of one's language skill ..."), which is a great idea! But we probably need a better question than that one. How about posting it as a new question? Maybe a self-answered one.
Being a non-native speaker, or maybe a native speaker who doesn't care much, the best answer that we can come up with is usually along the lines of "it just is".