say是说,例如 'I'm sorry,' he said... “对不起,”他说. 也可用于据说或据.报道 如The report says there are Forty-one people are said to have been seriously hurt.据报道有四十一人身负重伤.
speak是谈；演说；从某种观点来说,.也可表示说,但相比say要更正式一些.比如
I watched for an opportunity to speak. 我等待发言的机会
tell比较常用,口语里常用.tell story 如 I will tell you something.我将会告诉你一些事.
talk强调的是对话,感觉有两个人. 如 We need to talk . 我们应该谈谈