In one note from the documents, the FBI writes that Clinton said she did not know what the marking (C) — used to denote classified information deemed "confidential" — meant. When presented with an email chain using the (C) mark, Clinton "speculated it was referencing paragraphs marked in alphabetical order," according to the report.Everyone who handles classified information - that would be most of the military and a fair percentage of the Federal workforce - receives training on how to recognize classified material. The lowliest Seaman/Private/Airman is expected to know how this as part of their job. For Clinton, a woman who is a former First Lady, a former US Senator, and who was, at the time the Secretary of State, this is either inexcusable incompetence or blatantly dishonest. Dealing with classified information correctly is one of the BASIC job requirements of being SECSTATE, and something that she should have known how to do from her time as a Senator, and have gotten exposure to during her time as First Lady. This is roughly equivalent to your electrician not knowing that what DC and AC stand for.
Either she's too incompetent to be a good president, or too dishonest. Pick one, because there is no third option for her.