The /ð/ sound is spelled with <th>. The /ʃ/ sound is spelled with <sh>.
This is a contrast between a voiced dental fricative and a voiceless sibilant, fairly close together in the mouth. The separate sounds may be a problem for learners, but the contrast is so infrequent that it hardly matters.
One taboo word pairing is that/shat.Thanks to the rarity of the /ð/ sound, the mean density value is very low at 0.4%. The list makes 11 semantic distinctions, a loading of 85%.
other usher others ushers pother posher that shat thee she their share theirs shares thine shine though show those shows thy shy with wish withies wishes
John Higgins, Shaftesbury, November 2010.