Think Isn't Sink or Zink: French's Missing TH

Say 'think' and 'sink', then 'this' and 'zis'. If these pairs sound the same to you, you're substituting s or z for English θ and ð, since French simply doesn't have this tongue-between-teeth sound. To make it, let your tongue tip rest lightly between your teeth and push air through steadily — for θ it's just air (as in 'think'), for ð your voice buzzes too (as in 'this'). Try this: hold the tongue position, breathe out for two seconds without voice, then repeat with your voice turned on. Practice 'think/sink', 'this/zis', and 'both/bows', checking your tongue is actually between your teeth, not behind them. Give this two minutes today, slow and exaggerated. It will feel strange at first — that's normal — and it gets easier every time you practice it deliberately.