Speak 意思是“讲话”,强调说话的能力,方式和对象,即可以作及物动词也可以作不及物动词.作及物动词时,常用表示语言的名词或代词作宾语.例如：
Look!She is speaking in the classroom.
瞧,她在教室里讲话呢.
Do you speak English?
你讲英语吗?
Say 意思是“说”,强调说话内容.例如：
"What are you saying?""I'm saying nothing."
“你在说什么?”“我什么也没有说.”
Please say it in English.
请用英语说.
Talk 意思是“谈”,与介词to 或with 连用表示“与…交谈”,与介词about 或of连用表示“谈论…内容”.例如：
May I talk to you?
我可以和你谈一谈吗?
Xiao Li is talking with her teacher.
小李正和她的老师谈话呢.
What are you talking about?
你们在谈论什么呢?
Tell 意思是“告诉”,表示告诉某人某事.例如：
Please tell us the good news.
请告诉我们好消息.
I have something to tell you.