I'm convinced English is impossible to master. There are just too many corner cases.
That said, English is also a very abused language. Sadly these days, correcting someone leads to 'oh it's a living language and use determines definition', and more sadly, dictionaries play along with this idea.
This both makes the language harder to understand, and impossible to master IMO.
Its also impossible to master because what can be 100% correct in the US is wrong in India or in the UK or in Australia or New Zealand - all of who have English as their native language.
That said, English is also a very abused language. Sadly these days, correcting someone leads to 'oh it's a living language and use determines definition', and more sadly, dictionaries play along with this idea.
This both makes the language harder to understand, and impossible to master IMO.