This is what they ( said ) yesterday.
这是他们昨天所说的话.
Don't (talk ) in class,please be quiet.
不要在课堂上讲话.请安静.
speak 讲某种语言,如:speak English ,除此之外,一般为不及物动词.
talk为不及物动词.指连接不断的讲话,因此在课堂上讲话要用talk.
talk to/with sb.跟/同某人讲话
say为及物动词.可说say sth.如第一空有宾语what,因此要填said.
tell为及物动词.但一般的搭配为:tell sb./sth.如:tell a story/lie
tell sb.about sth./ tell sb.sth.