The document discusses the use of "by" and "with" to express how something is done. "By" is used to express how something is done through means of transportation and communication, such as by plane, by train, by mail. "By" is also used with a gerund to express how something is done, such as turning off the TV by pushing a button. "With" is used to express how something is done using instruments or parts of the body, such as cutting down a tree with an ax or sweeping the floor with a broom.