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.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