I don’t like to use the word “corrupted”. The Book of Mormon tells us that many “plain and precious parts” have been removed from it.
As for which parts they were, we know of some of them though modern revelation, as previously mentioned; and the rest remain to be revealed.
Again, not corrupt, but incomplete.
How do we know? Because the Lord has revealed it to us, and there are also internal evidence within the Bible to support it.
In the same way that we know the non-restored parts are correct—by the revelations of the Holy Ghost.
Because we don’t believe it is “corrupt”. That is the language that you like to use, not us. Bits have gone missing from it, which is not the same as saying it is corrupt.
