
Originally Posted by
Linguist__
Well, considering here in Britain voiceless 'th' being realised as 'f' is increasingly common, in a few generations the student's incorrect pronunciation will be correct. Already, several dialects in Britain have 'f' as standard, and no doubt the others will catch up soon.
It's a good thing, I think. Dental fricatives are rare in languages, and even in English, it is one of the last phonemes children learn. In fact, it is the last sound to be learnt before the voiced post-alveolar fricative ('measure'), but this sound is rare in English, so it could be assumed that the dental fricatives are indeed the last sound to be learnt. Also, as I said, a lot of people say 'f', so some children never learn it!
As a speech therapist, a child who says 'f' for 'th' wouldn't be given speech therapy. The fact is, if their communicative environment uses 'f' for 'th' then it is what the child should be expected to do.