That McGurk effect…
Both my kids say f instead of th. Trying to talk them through it, it turned out that they couldn’t really hear the difference. Then we did some experimenting. The sounds seem different in our own heads (maybe because of acoustics or maybe because we are making a different shape) but my wife and I were both differentiating the sounds based on recognition of the word and watching the speaker’s mouth. Listening to the sounds only, with our eyes closed, we realised that the kids have a point and it was very difficult to tell the difference.
There’s a similar thing going on with written language and spelling. In Welsh, the aspirate ‘th’ (as in three) and voiced ‘th’ (as in there) are different letters: th and dd respectively. But when trying to explain this to an English person he was unable to appreciate there was a difference in the two sounds, because the fact that they were written the same in his first and only language had fixed the concept in his brain. Even though he could pronounce both sounds at will. Likewise, I knew a Finnish person called Ville. Some people called him Ville and some Wille – so I asked him which was correct and he said ‘either – it’s the same sound’. In Finnish apparently it’s a question of accent, as it is in England with words like ‘but’ being pronounced differently in the North and South. They are different vowels but we perceive them as the same even though we can all hear the difference clearly.