The /ð/ sound is spelled with <th>. The /j/ sound is spelled with <y>.
This is a contrast between a dental fricative and a palatal semivowel, both voiced and fairly close together in the mouth, but the contrast does not cause problems.
The contrast only occurs initially and is very rare since only a few words, mainly function words being with /ð/.
thee ye then yen their Yare there Yare they yea