th sound

The consonant sounds of English are normally categorized as plosives,affricates, fricatives, nasals and approximants. Is that what you are asking about?

The two sounds you mention are fricatives.

thanks for your quick reply,but what I wanted to know is when a word is pronounced /ð/ and /?/ ? as a forign language ,English grammar for our students needs the "why" to know the rule and to better understand.
In a test for example,if they are given words with "th",and asked to classify them : /ð/ and /?/ ,they won't know not only because they aren't allowed to use dictionary but also they c

In conjunctions, determiners and pronouns, word-initial th is generally /ð/: though. the, that, them. In other words, it is generally /?/: thick, thread, thermal..

In other positions, there are no clear guidelines

thanks a million for your answer

From the LongmanPronunciation Dictionary (Third edition, 2008):

In the middle of the word (provided that th is not at the end of a stem) the pronunciation is generally

? in words of Greek or Latin origin as in method, author, ether

yeah indeed it's helpful only if one knows the origin of the word,alas it's not the case