Might I ask to what degree British National Corpus and American Corpus can be treated as a reliable source? Do all sentences in them are good English?
Could I ask native English teachers to answer these questions?
Thank you very much.
There will be many mistakes in the language used because that reflects the nature of any language. However, they are both large enough to give an interesting and relevant picture of usage. I use these sites all the time to access them:
[Davies/BYU] Corpus of American English
[Davies/BYU] BYU-BNC: British National Corpus
The BNC, if anything, has a bias towards conventional forms, especially formal writing. The ANC has a lot more spoken language in it.