Read this sentence out loud: I think this is thirty-three thieves. If it came out closer to I sink zis is sirty-sree sieves, or I tink dis is tirty-tree tieves, you've just diagnosed your accent in a single line.
French has no /θ/ and no /ð/. The two English TH sounds are dental fricatives produced with the tongue tip between the teeth, and that mouth shape is not in the French inventory at all. The brain does what brains always do: it reaches for the closest French neighbour. That neighbour is /s/, /z/, /t/, or /d/ depending on which French you grew up speaking, and the substitution is so consistent that linguists can predict which one will land based on your dialect alone.
Most TH guides point at a tongue diagram and call it done. You probably already know the tongue is supposed to come out a bit. What you don't know is which French sound is hijacking the move every time you go fast, why /θ/ and /ð/ are actually one drill instead of two, and why the /h/ in hello is the same problem in a different costume.
The thirty-three thieves test
Open your phone, hit record, and read this aloud at normal speed: Thirty-three thieves think this thin path is for them. Play it back.
If the TH words come out with /s/ and /z/, like sirty-sree sieves sink zis sin pas is for zem, you're producing what's called the European French substitution pattern. If they come out with /t/ and /d/, like tirty-tree tieves tink dis tin pat is for dem, you're producing the Quebec French pattern. A 2023 paper in Frontiers in Language Sciences attributes this split to differences in each variety's underlying feature hierarchy, not to laziness or imperfect teaching. The substitution is structural.
One useful nuance from the Fonds de recherche du Québec summary of work by Paul John: French speakers don't fail systematically. They alternate. The same speaker says think correctly in one sentence and sink in the next. That means the muscle move is already in your repertoire. The work isn't learning the move from scratch. It's making the move automatic so it doesn't disappear at conversational speed.
How to pronounce the TH sounds: one tongue position with a switch
The voiceless TH in think, three, bath is /θ/. The voiced TH in this, the, mother is /ð/. Same tongue placement: tip lightly between the teeth, air flowing around it. The only difference is whether the vocal cords are buzzing.
Put a finger on your throat and say this. You should feel a steady vibration through the whole word. Now say think. The vibration only kicks in on the vowel; the opening TH is silent friction. Rachel's English describes the move as "the very tip of the tongue comes through the teeth," with the tongue staying relaxed rather than pressed. Tense tongues produce sloppy fricatives.
Four word pairs drill the voicing contrast directly:
- thigh /θaɪ/ vs thy /ðaɪ/
- bath /bæθ/ vs bathe /beɪð/
- breath /brɛθ/ vs breathe /briːð/
- mouth (the noun) /maʊθ/ vs mouth (the verb, "to mouth the words") /maʊð/
If you can switch cleanly between bath and bathe with your finger on your throat, you've got the move. If both come out as /bas/ and /beɪz/, your tongue isn't reaching the teeth at all. If they come out /baθ/ and /beɪθ/, you have /θ/ but the voicing isn't engaging. Different bug, different fix.
Match your substitution to the right minimal-pair row
If your think keeps landing as sink, the row to drill is /θ/ → /s/, not all six possible substitutions at once. Find the row that matches your recording and start there.
| Your substitution | Minimal pairs to drill |
|---|---|
| /θ/ → /s/ | think/sink, thin/sin, thigh/sigh, thank/sank |
| /θ/ → /t/ | three/tree, thick/tick, thank/tank, both/boat |
| /θ/ → /f/ | three/free, thin/fin, deaf/death |
| /ð/ → /z/ | breathe/breeze, then/Zen, they/Zay |
| /ð/ → /d/ | they/day, then/den, those/doze, this/dis |
English Club has audio for most of these pairs, organized by exact substitution type. That's unusual; most minimal-pair sites mix everything together. The /d/ vs /ð/ page is especially useful for Quebec French speakers, and the /s/ vs /θ/ page is the one to bookmark if your think keeps landing as sink.
Pick three pairs from your row. Say each pair five times slowly with your finger on your throat. Stop when you can hear the contrast yourself, not when the timer runs out.
The paper-bite drill
Tear a strip of paper about two centimetres wide. Hold it lightly between your front teeth so it sticks straight out, like a tiny flag. Now say thirty-three thieves think this is thin five times.
The paper falls early if your tongue retreated to /s/, /t/, /z/, or /d/ at any word. None of those sounds need the tongue between the teeth, so the paper drops the moment you collapse to your French default. When the tongue tip emerges past the teeth on every TH, the paper stays put through the whole sentence.
Add a mirror and watch your tongue. If you can't see the tip emerge on think, the move never happened, no matter what you felt. The paper test catches what your ears miss; the mirror catches what the paper misses.
Try it in Conversa
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Try Conversa FreeThe /h/ that rides shotgun
French has no phonemic /h/. The distinction between h muet (mute h) and h aspiré (aspirated h) is purely about whether the word allows liaison and elision, not about an actual /h/ sound. Neither one is pronounced. So when a French speaker meets English /h/, two opposite errors are equally available.
Error one: dropping it. I 'ave a 'eadache. 'Otel, 'appy, 'ello. The mouth doesn't know to add the puff of air at the start of the word, because no French word ever asked for one.
Error two, less common but real, is overshooting in the other direction once you've been corrected. Some learners produce H-it's a nice day or h-everything, sometimes with a small glottal kick at the start of a vowel where English would just glide in. Treat it as a side effect of trying too hard, not the main problem.
Pair the /h/ fix with your TH session. The line Harry has a happy hippo trains the puff of air at the start of every word. Hold your hand in front of your mouth; you should feel a small breeze on every /h/. No breeze means no /h/. With practice, the same attention pattern that catches a missing TH starts catching a missing /h/, because both are friction sounds your French ear didn't grow up sorting. If you've already worked through scenario-specific French false friends on the vocabulary side, this is the pronunciation companion piece.
Connected speech: when /ð/ disappears (don't worry about it yet)
Native English speakers do not pronounce the the same way every time. In I think that this is fine, the th in that and this gets shorter and softer, sometimes nearly inaudible. The function-word reductions catalogued by Rachel's English are a real feature of fluent speech.
The trap for a learner: hearing I think 'at 'is fine in casual native speech and thinking the TH is optional. It isn't optional. It's reduced, which is a different thing. Reduction is a privilege you earn after the underlying sound is clean. If your isolated this is still landing as /zɪs/ or /dɪs/, you haven't earned the casual version. You'd be skipping a sound you can't yet make.
Drill in slow, isolated form first. Connected-speech reduction sorts itself out automatically once the base sound is real. Trying to fake the casual version while the base is still wrong just adds a second error on top of the first.
Two weeks, five minutes a day
Work the substitution row from your recording first. Three pairs, five repetitions each, with the paper test on a single sentence at the end of every session. After the third or fourth day you'll start hearing your own tongue retreat in real time, and that's the cue to layer in the voicing pairs from earlier (bath/bathe, breath/breathe) with a finger on your throat. Once bathe sounds different from bath without thinking, add the Harry has a happy hippo line to catch the /h/ that travels with the same problem.
The last few sessions are for shadowing. Pick one minute of an English audiobook and play it on loop, narrowing your attention to TH and /h/ only. Let every other sound go. The narrowing is what makes the practice stick; trying to fix every sound at once is what makes it not. The same narrowing principle works for the Spanish single R and trilled RR if you're working a second language at the same time. If you want a daily conversation partner that won't get bored of correcting think fifty times in a row, Conversa is the AI conversation partner we built for exactly that kind of repetition. The drill above works on its own with any honest mirror and a willingness to record yourself.
When you can say I'd rather breathe than think about this without your tongue retreating once, you've made the muscle move automatic. The accent stays, and that's fine. Accents are not errors. The substitution stops, which is the part that mattered.