Hi, Beth, it seems you really into these "finding differences" questions, haha! 说、讲are vt. e.g. I 说 Chinese. or I 讲 Chinese. 说、讲are the same when it comes to "speak". But 讲 has another meaning: 讲礼貌or讲文明, they both mean "be polite", such expressions are mostly seen on some broad banners in prelimentary schools. Such expression is very old-fashioned, so don't say it, understanding it is enough. 说话and 讲话are vi. e.g. Do not 说话or 讲话in the classroom! (Keep Silent!) 讲话has another meaning as "deliver a speeach, the person who 讲话must be someone in higher positions, like a governor, or a boss etc. And it happens in very grand occasions. e.g. Before the Olympic Games' opening ceremony begins, let's welcome our chariman讲话。 Besides,讲话is also noun. e.g. Governor's 讲话 indicates that......(often seen in newspaper reports)

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说话就是一般人与人之间谈话..2个或多个人交谈(说话：2 or more people chat) 讲话就是一个人讲,其他人不说话.其他人听（讲话：one people speech, other people just listen not speak)

they can all used as verb ,beside that 讲话 and 说话 can used as none .for example ,ta 讲话/说话 hen kuai ,means the speed of he speak is fast ; ta 讲话/说话 rang ren sheng qi,means the content that he talked is provoking . i think the diffrence is the situation that they used .

说&讲 are all verbwhich mean speak, the former one we usually use it in a imformal way, while we use the latter in a morfmal way, expecial for government officer, profeoosr, interview etc. 说话&讲话 are all noun which mean the words or sentences you speak out, the usage and difference are all the same as above. On the side, 说 can insteade of some whole sentence when we spoken it in oral Mandarin, i.e. "Speak!"in english