In my article on French vowels, I explained how different languages have different phonemes, giving the example of how native French speakers have trouble with the “th” sounds while learning English. In fact, these sounds are quite unusual cross-linguistically, appearing in only about 4% of the world’s languages —so they give trouble to a huge number of English second-language learners, not just the French. I’m a native English speaker, so I don’t remember learning to pronounce these sounds as a toddler —but I thought it would be an interesting phonetics exercise to try and describe their pronunciation. I refrained from publishing this until I heard back from one of my French friends that this was helpful to them, so hopefully it will be to others as well.
The first thing to understand is that there are actually two ‘th’ sounds in English. Although most English speakers aren’t aware of the difference, they can tell between the words “thigh” (part of the leg) and “thy” (an old-fashioned way of saying “you”).
The ‘th’ in “thigh” is /θ/ in IPA.
The ‘th’ in “thy” is /ð/ in IPA.
I’ll explain the difference later, but let’s start with /θ/
When non-native speakers don’t know how to make this sound, they usually produce a /f/ or a /s/ instead: eg, they’ll sometimes say “fink” or “sink” instead of “think.” This isn’t arbitrary: they choose these sounds because they are both fricatives, just like /θ/. A fricative is a sound that’s made by making a closure between two parts of the mouth and letting out a small stream of air.
When you make an /s/, you touch the tip of your tongue to the alveolar ridge (the little bump on the top of your mouth behind the teeth). Hold the /s/ in “yessssssss—”. You will feel how you are releasing a little bit of air between your tongue and the roof of your mouth.
Now try the same thing with /f/. You make an /f/ by touching your front teeth to your bottom lip. Now if you hold the /f/ in “stufffffff—”, you’ll feel how you are letting the air pass between your lip and your teeth. Notice that a little bit of air also goes around the sides of your teeth.
/θ/ is the same, only instead of making the closure with your teeth and your lips, you make it with your teeth and your tongue. To make the /θ/, touch the tip of your tongue to your two front teeth. Then blow a little air between your teeth and your tongue, just as you do to make an /f/, just substituting the lips for the tongue. The opening of your mouth should not be too wide, or else all the air will go around and not between the tongue and the teeth. However, you should have a little opening at the sides of your mouth, or else the sound will not be loud enough. Once you’re doing this, you should be able to transition to saying “—thhhhhhhhigh.”
/ð/ is exactly the same, only you also vibrate your vocal folds. To understand what this means, put your hand on your neck. Then hold a /v/ sound as in “slavvvvvvvvvvvv—” you will feel a vibration in your nest. Now do the same thing but holding /f/, like in “struff”. You’ll notice that you don’t feel the same vibration in your neck when you make the /f/ sound.
Linguistics call this property voice. Languages differentiate between sounds depending on whether they are voiced or not. /v/ is the voiced version of /f/, /z/ is the voiced version of /s/, /d/ is the voiced version of /t/, etc.
To learn how to control the voicing, try to transition between a /s/ sound and a /z/ sound. They are exactly the same except for the voicing. Keep your hand on your neck and go “ssssssss zzzzzzzzs ssssssss zzzzzzzz.” As you do this you can become conscious of how your vocal folds are vibrating.
To make a /ð/, try and hold a /θ/. A good word that ends with this is “bathhhhhh—”. Now, as you hold the /θ/ sound, start vibrating your vocal folds, the same way you did as you transitioned from /s/ to /z/. If you do it, you’ll be making the /ð/ sound and you can transition to “—thhhhhy”
Some words that use /θ/ (voiceless): think, thanks, thunder, thirsty, thumb, throw, theory, throat, bath.
Some words that use /ð/ (voiced): this, they, father, weather, bother, bathe.
As is often the case in English orthography, there is to my knowledge no way to determine the sound from the spelling of the word. However, now that you know the IPA for each sound, you’ll at least be able to tell from the pronunciation guides in dictionaries.