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