speak强调说话的能力,方式和对象,不强调说话的内容.作为及物动词时,后面一般跟表示语言的名词或代词.而作为部及物动词时,后面一般用介词to.
eg：The young man is speaking Japanese.
The teacher is speaking to his class.
talk强调“谈论”、“交谈”、“谈话”,后面常跟介词to或with搭配,表示“与.交谈”,与介词about或of搭配,表示所谈论的内容.但一般较少用of.
eg：May I talk with you?
What is he talking about?
tell强调“告诉”,“讲述”,一般用作及物动词.
eg：I tell him my home address.
My grandpa often tells me stories at night.