th有两个发音,一个是「θ」什么时候发「ð」

　　(1)th位于词首时,可以从词性上判别其应有的读音.

　　在名词、动词、形容词和数词中th发清辅音.

　　如：thing,theatre,thunder,thermos,Thursday,theory,theme,throat,thread,think,thank,thrive,thicken,thirteen,thirty,third,thousand,thick,thirsty,thoughtful,thorough在代词和一些功能词中th发浊辅音.如：them,their,theirs,there,the,than,then,though,thus,therefore,they.只有介词through是个例外.

　　(2)th位于词尾（或音节之尾）时,其读音多数是清辅音,如：bath,breath,tooth,等.

　　只有在少数单词中,如：with,smooth中读浊音.

　　(3)th在词中并且后面接er时,或者最后一个字母是不发音的e时,通常也发浊辅音.

　　例如：feather,weather,whether,leather,further,father,bathe,breathe,southern,northern,either等.在其他情况下发清辅音.例如：author,faithful,method,nothing,anything,healthy,wealthy.

　　试读下列单词:mother,father,another,brother,health,wealth

　　希望可以帮到你=）

　　http://zhidao.baidu.com/link?url=9KLe0fE6IQC43CJLdzP3gVFxeDDWa5ZG4a6nsdaplDvs3-5RTXKs3aiSnQ6kDseUGMazrpllUF5yHIYAbbWMsK