speak强调开口说话，发言的动作。
Do you speak Chinese?
你说汉语吗？
talk强调双方谈话，后可搭配介词to,with,about等。
Jim is going to talk about English names tomorrow.
吉姆打算明天谈谈英文名。
say强调说话内容，指有连贯性的说话。
"I'm so tired."she said
她说“我太累了”。
talk指把一件事传达给别人或讲述一件事情，一个故事等。
When I was young ,my mother often told me a story in the evening .
在我小的时候，妈妈晚上经常给我讲故事。