1. speak强调单方的“说”或“讲”，一般用作不及物动词，要表示“对某人说(某事)”，可用 speak to [with] sb (about sth)。如：
Please speak more slowly.请说慢一点。
I spoke to [with] the chairman about my idea.我跟主席说了我的想法。
2. talk强调双方“交谈”，一般用作不及物动词，表示“同某人谈论(某事)”，可用 talk to [with] sb (about sth)。如：
He was talking to [with] a friend.他在同一位朋友谈话。
What are they talking about?他们在谈论什么?
注：有时可用作及物动词，表示“谈论”、“讨论”等，但此时通常只与个别名词连用。如：
talk business谈正事 talk nonsense [rubbish]胡说八道
3. say强调说话内容，一般用作及物动词。表示“对某人说”，可用 say to sb。如：
Did you say anything (to him)?你(对他)说了些什么?
He said (that) he wanted to go.他说他想去。
注：以下句型值得注意：
据说他病了。
正：It is said that he is ill.
正：He is said to be ill. (from www.yywords.com)
4. tell表示“告诉”，可接双宾语，双宾语易位时用介词 to引出间接宾语。如：
I told him my name.我把名字告诉了他。
He told his parents the good news. / He told the good news to his parents.他把这个好消息告诉了他父母。
注：还可接不定式的复合结构作宾语。如：
Tell her to come at once.叫她马上来。
He told the children not to play in the street.他叫孩子们不要在街上玩。
另外，有时与介词 from连用表示“区分”、“辨别”等。如：
I cannot tell which is which.我分不清哪个是哪个。
Can you tell true friends from false friends?你能分清真 假朋友吗?
5. 注意习惯表达：(from www.yywords.com)
(1)表示“演说”、“演讲”、“发言”等，一般用 speak。如：
Who is going to speak at the meeting?谁将在会上发言？
(2)表示“讲故事”、“说谎”等，一般用 tell。如：
He often tells us interesting stories.他经常给我们讲有趣的故事。
(3)表示“说某种语言”，一般用 speak (及物)。如：
Do you speak French?你会说法语吗?
注：偶尔用动词 talk表示“说”某种语言，但很少见。
(4)在某些搭配中，有时可有多种表达。如：
say [speak] a word说句话 say [tell, speak] the truth讲真话