(1) In word-initial position <th> is pronounced
- /ð/ if it's a function word such as the, that, this, than; it's debatable whether you can call adverbs like then, there, thither, thence (the latter two also with voiceless initial <th> in General American) functions words;
- /θ/ if it's a content word such as three, thick, thank, think, theatre, theme.
- /ð/ if the word is of Germanic origin: brother, father, worthy, weather, ...;
- /θ/ if the word is of Greek, Latin or non-European origin: method, author, cathedral, Athapascan, ....
- exclusively /ð/ in some cases: to mouth, eth (= letter name), with, smooth;
- either /ð/ or /θ/ in booth, outwith, herewith, ...;
- /θ/ only in bath, birth, month, north, ....