A sure sign of ignorance is the belief that we know a language “perfectly.” If you accept that idea you will become lazy and careless and you will not improve the way you communicate either orally or in writing. You will get nowhere fast.
Not a day goes by that I do not learn a new word, or discover a grammatical flaw in my Spanish or English. Often I find new meanings to old words or idiomatic expressions that enrich my languages. I know now that to fully possess a language is a chimera, a delusion, a real impossibility, because it is a slippery, living, chameleonic, fickle creature that never stands still.
Language is a tool that must be kept up to date all the time.