See the difference
For th, the tip of the tongue comes forward to the teeth. For s, the tip stays behind them and the front of the tongue rises close to the ridge just behind your top teeth. Both let air through with no voice. The only difference is where the tongue is, and that is why your ear can't always tell you which one you said.
Try it in a mirror
- Look in a mirror or your phone's camera. Say "sss". You shouldn't see your tongue.
- Keep blowing and slide the tongue forward until the tip just touches your top teeth. You should see a little of it. The sound gets softer and flatter.
- Hold that for two seconds, then add a word: "thhh-ing".
- Now say "thing" at normal speed, then "sing", and watch the tongue appear and disappear.
You don't need to push your tongue far out. Too far out is a common overcorrection, and it makes the sound slow to say inside a sentence.
Pairs to practice
- sink
- thicksick
- mouthmouse
- worthworse
Does practice work?
Yes. Th is well documented as a Chinese-speaker feature: voiceless th becomes s, and voiced th becomes z or d. In a randomized study of 70 Chinese graduates, training that combined listening to many voices with seeing the speaker's face improved th, and teacher-led practice did not. The lips and tongue tip are visible on this sound, so watching a mouth, including your own, is useful.
The other th
English has two: the voiceless th in "think" and the voiced th in "this", "they" and "mother". The tongue is in the same place, and your throat buzzes for the voiced one. Chinese speakers often turn it into d or z, so "they" comes out as "day" or "zay". Put a finger on your throat and check for the buzz.
What to do next
Reading how a sound is made helps, but a page can't tell you whether your own tongue got there. In Language Boss, every sound you say in a scene is scored after the call, and you're told what it came out as. X-ray Mouth draws the tongue and lips over your own cheek while you say a phrase to the camera, so you can see what to change. X-ray Mouth is free.
A sound isn't fixed when you can say it in a drill. It is fixed when it still holds in a real conversation, when your mind is on something else. That is the test we use: Language Boss counts a sound as fixed only when it holds in later open speech.
Questions
Why do Chinese speakers say s instead of th?
Mandarin has no th sound, so the mouth reaches for the closest one it knows. S is close because the air and the lack of voice are the same. Only the tongue position differs.
Is it a problem if I say sink instead of think?
Usually people work it out from the context, so it is rarely the first thing to fix for being understood. But it is one of the most noticeable features of a Chinese accent, and it can change a word. If you want to sound the way you hope to, it is worth a few weeks of work.
How long does it take to fix th?
Most people can make the sound the first day, with a mirror. Using it in fast speech without thinking takes much longer, because it has to replace a habit. There is no reliable number, so measure it on your own recordings.
