Reference page

Voiced or voiceless

TH

Words that start with ð:

the, that, their, there, they, than, these, them, though, thus, thou/thee/thy/ thine, thence/thither

Words that end with ð:

with, mouth (verb), loath, smooth, betroth, bequeath

anything spelled -the:

breathe, bathe, clothe, blithe, writhe, sheathe

Words containing ð:

Words containing the above examples

e.g. although, nonetheless, withdraw, mouthing, clothes

Words ending [ð əm]:

rhythm/rithm: rhythm, logarithm, algorithm, rhythmic, etc
fathom

Words ending <-ther>:

other, mother, brother
weather, leather, feather, heather
whether, tether, nether, Netherlands
father, rather, lather
further
gather
bother

Exception: the Greek word (a)ether is θ

Words which are θ in the singular but (usually) ð in the plural:

paths, baths, oaths, youths, truths, mouths, wreaths, sheaths

Remaining words which are ð:

worthy (but not worth); smithy (but not smith)
southern, northern, heathen (but not south, north or heath)
brethren (brothers, usually metaphorically as in monks; old fashioned)

Welsh names

ð is written as <dd> in Welsh. You may encounter this in English as names (e.g. Dafydd), placenames (e.g. Pontypridd) or cultural references (e.g. eisteddfod, a Welsh festival).

You will be understood if you mispronounce these words with a [d] instead of a [ð]; many English Brits do. But better if you can pronounce them the way the Welsh community does.