speak：可作不及物动词,表示说话；也可作及物动词,表示一种能力.
tell:告诉,常与介词about,to连用.
常用句型：tell sb about sth; tell sb to do sth
talk：侧重说话,多与to,with,about连用.
e.g.Can you speak English?你会说英语吗?
Please tell me about his situation.请告诉我关于他的情况.
Tell him not to smoke.告诉他不要吸烟.
I'd like to talk to/with her.我喜欢和他交谈.
Don't talk about the bad weather.不要谈论这个坏天气.