The /ð/ sound is spelled with <th> or <the>. The /z/ sound is spelled with <z>, <zz>, <s> or <se>.
This is a contrast between a dental fricative and a sibilant, both voiced and close together in the mouth. It is a problem for many learners including Dutch, French, German, Russian, Chinese and Thai. It is worth noting that the sequence /ðz/ is often assimilated to /z/, so that "Put your clothes on" sounds like "Put your close on", which may lead to spelling mistakes.
Thanks to the rarity of the /ð/ sound and the commonness of the /z/ sound, the mean density value is very low at 0.4%. The list makes 29 semantic distinctions, a loading of 73%.
bathe baize bathe bays booth boos booth booze breathe breeze breathed breezed breathing breezing clothe close clothed closed clothing closing Hythe hies Hythe highs lathe lays lathe laze lithe lies loathe lows seethe seize seethed seized seething seizing scythe size scythed sized scything sizing soothe sous swathe sways teethe teas teethe tease teethed teased teething teasing tithe ties thee zee these zees then Zen thither zither with whiz withies whizzes withered wizard writhe rise writhing rising
John Higgins, Shaftesbury, April 2012.