My father speaks very well English,but he knows little Japanese.
这个句子是个有but连接的复合句子,相当于汉语里的并列转折从句,也就是说：My father speaks very well English是一个句子,而he knows little Japanese又是另外一个句子.
常用的连接两个句子的词,我们称为 “连词",常用的有:and ,but ,however,nevertheless,because,in that等.
有什么不懂得,