The /ð/ sound is spelled with <th>. The /w/ sound is spelled with <w>.
This is a contrast between a dental fricative and a bilabial semivowel, both voiced but well apart in the mouth. The contrast does not cause problems.
The contrast only occurs initially and is quite rare since only a few words, mainly finction words being with /ð/. The mean density value is very low at 0.1%. The list makes 25 semantic distinctions, a loading of 93%.
thee we thee wee their ware theirs wares their wear theirs wears their where then wen then when thence whence there ware there wear there where thereabouts whereabouts therefore wherefore therewithal wherewithal these wees they way thine whine thine wine thither whither though whoa though woe those woes thou wow thy why thy Y
John Higgins, Shaftesbury, November 2010.