This writing style is archaic in most contexts. Either the quote is very old, or it was written like that for poetry or emphasis. In modern English it'd typically be written like this: "Do not say everything (that) you know and do not believe everything (that) you hear." I wrote "do not" instead of "don't" to have has similar emphasis. The sentence style isn't wrong, but extremely uncommon nowadays. "Do not say everything you know" - This basically means "be tactful when talking" "Do not believe everything you hear" - This basically means "don't always believe what people tell you".

As another member said, this is an example of archaic English (older English that is now used only on special occasions). Our President Kennedy was famous for saying: "ASK NOT WHAT YOUR COUNTRY CAN DO FOR YOU, ASK WHAT YOU CAN DO FOR YOUR COUNTRY." In everyday speech, we would just say: "Don't ask what your country can ...."

Don't say that you have known the whole things and dont believe all of the things you hear