Speak比较庄重，表示某人(即讲者)在一段对话中所说的话较其它人多。
举例：
The Principal speaks to us every morning at the assembly.
Tom, your boss wants to speak to you.
Talk不及speak庄重，而且多用于说话，少用于书写。 Talk表示两个或更多的人在对话。
举例：
The students were talking loudly in the classroom, in the very presence of the teacher.
当宾语是一种语言时，要用 speak 不用 talk 或 say：
正确： Victor speaks fluent Putonghua.
错误： Victor talks fluent Putonghua.
错误： Victor says fluent Putonghua.
在书面英语， talk 极少用作及物动词，除非在 talk 后接用以下的宾语：
Stop talking nonsense.
They are talking business again.
若不是接用上列的宾语，talk 后必需接用前置词 about。
举例：
正确： Betty and Susan are talking about the trip to London.
错误： Betty and Susan are talking their trip to London.