The two TH sounds, and how to finally say them
The letters TH look like one thing, but they're two completely different sounds in English: the voiceless /θ/ in think and the voiced /ð/ in this. Almost no other language has either one, which is why TH is one of the most commonly avoided sounds in the world, quietly replaced with a /t/, /s/, /f/, /d/, /z/, or /v/ depending on where you learned English. The good news is that both sounds use the exact same tongue position — you just need to learn it once.
One mouth shape, two sounds
For both TH sounds, the tip of your tongue goes forward until it lightly touches the bottom edge of your upper front teeth, or pokes just barely between your upper and lower teeth. Air is pushed out over the tongue, past that light contact, creating a soft, continuous hiss. That's the entire shape. There's no vibration in the throat for /θ/ — put your fingers on your throat and say think, and you'll feel nothing hum. Do the same for this, and you'll feel your throat buzz the whole time you hold the sound. That buzz is voicing, and it's the only difference between the two.
This means the fix for both sounds is the same physical habit: get your tongue tip out to your teeth instead of pulling it back into your mouth to make a /t/, /s/, /d/, or /z/, which is what most substitutions do. You can watch the tongue position for both sounds side by side on the sound reference before you try to hear the difference by ear.
Why the substitution happens, and which one you probably make
Nearly every substitution comes from a learner's first language reaching for the closest sound it already has, and which one you reach for is a strong tell of your language background. Speakers of French, German, and many Slavic languages tend to replace /θ/ with /s/ and /ð/ with /z/, so thin becomes sin and they becomes zey. Many speakers of Caribbean English, some West African languages, and some varieties influenced by them use /t/ and /d/ instead, so thin becomes tin and they becomes day. Speakers of Russian and several other languages often reach for /f/ and /v/, turning think into fink and this into vis.
None of these are wrong reflexes — they're the nearest available sound in your existing inventory, and native listeners will usually still understand you. But each substitution collides with a real English word. Thin, tin, sin, and fin are four different words. So are they and day, and than and ban is close enough that stress and context are doing a lot of quiet work to save you when TH slips.
Minimal pairs to test yourself
Say each pair below and check that your tongue actually moves between the two words, not just your intention: thin / tin, thin / sin, thin / fin, thick / tick, thought / taught, path / pass, bath / bass. For the voiced sound: they / day, then / den, breathe / breed, clothing / cloning is a fun stretch, but this / dis- as a prefix and writhe / ride both make the point cleanly.
Record yourself saying five pairs and listen back, or better, run each sentence through SayNative and look at the sound-by-sound breakdown on the phonetic sheet — it will flag every TH and mark which of the two it is, so you can see exactly where a word needs the tongue-forward sound instead of the substitute you've been using.
Why /ð/ hides in plain sight
Here's something that surprises most learners: the voiced /ð/ is actually one of the most frequent consonant sounds in spoken English, because it sits inside almost every function word — the, this, that, these, those, they, them, their, there, then, than, though. You cannot get through a single sentence of English without it. The voiceless /θ/, by contrast, mostly shows up in content words: think, through, thank, nothing, math, truth.
That distribution matters practically: if your /ð/ is weak, every sentence you speak will sound slightly off, because the alone might appear a dozen times in a short paragraph. It's worth drilling the, this, and that in isolation until the tongue-forward touch becomes automatic, since fixing those three words fixes the sound in most of the sentences you'll ever say.
A mirror drill that works
Stand in front of a mirror and say think slowly, watching your tongue tip. If you don't see a flash of tongue between or against your teeth, you're making a /t/ or /s/ instead. Exaggerate it at first — let the tongue tip stick out visibly — then gradually pull it back to a smaller, more natural touch once the sound is reliable. Do the same with this, checking for the throat buzz with your fingers at the same time.
Once both sounds feel automatic in isolation, move to short phrases that mix them: this thing, that's the truth, think about that. Reading these on a SayNative-generated sheet, with the two TH marks shown separately, gives you a visual anchor while your mouth is still learning the new habit — after a few weeks most learners stop needing the mirror at all.
If your substitution habit is deeply automatic, expect the first attempts to feel exaggerated or even a little silly, and expect the sound to disappear again the moment you stop concentrating and start focusing on meaning instead of pronunciation. That's normal for any new articulation, not a sign you're doing it wrong. Building the habit into short daily phrases, rather than only reciting isolated word pairs, is what eventually lets the tongue-forward touch survive into unscripted conversation.