(1) th位于詞首時，可以從詞性上判別其應有的讀音。在名詞、動詞、形容詞和數詞中th發清輔音。如：thing, theatre, thunder, thermos, Thursday, theory, theme, throat, thread, think, thank, thrive, thicken, thirteen, thirty, third, thousand, thick, thirsty, thoughtful, thorough 在代詞和一些功能詞中th發濁輔音.如： them, their, theirs, there, the, than, then, though, thus, therefore, they。只有介詞through是個例外。

(2) th位于詞尾(或音節之尾)時，其讀音多數是清輔音，如：bath, breath, tooth, 等。只有在少數單詞中，如：with, smooth 中讀濁音。

(3) th在詞中并且后面接er時，或者最后一個字母是不發音的e時，通常也發濁輔音。例如：feather, weather, whether, leather, further, father, bathe, breathe, southern, northern, either等。在其他情況下發清輔音。例如：author, faithful, method, nothing, anything, healthy, wealthy。 試讀下列單詞: mother, father, another, brother, health, wealth