There are no rules about when a word contains either a voiced th sound or a voiceless th sound.
However, there are certain regularities that may help you. They can be categorised into three groups: th at the beginning of a word, th in the middle of a word and th at the end of a word.
1. Th at the beginning of a word:
- In almost any case, the th is a voiceless /θ/ as in threat, thought, think, throw, ….
- Function words usually begin with the voiced /ð/ as the pronouns they, them, their, the, this, that, these, those and the adverbs and conjunctions then, there, than, thus, though, therefore, therbey, thereafter, ….
2. Th in the middle of a word:
- Most English words have a voiced /ð/ in the middle, when there is a th, as in either, father, mother, brother, rather, further, together, weather, whether, ….
- Most loan words have a voiceless /θ/ in the middle, when there is a th, as in cathedral, enthusiasm, ethics, mathematics, lethal, method, mythical, ….
3. Th at the end of a word:
- Nouns and adjectives usually end in a voiceless /θ/, when there is a th, as in bath, cloth, breath, tooth, teeth, ….
- Verbs usually end in a voiced /ð/, when there is a th, as in breathe, loathe, soothe, writhe, ….