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/. The mean density value is very low at 0.1%. The list makes 5 semantic distinctions, a loading of 100%.
thee ye then yen their Yare there Yare they yea
John Higgins, Shaftesbury, November 2010.