Preventing Extinction

There is a sound which is slowly become extinct from our spoken language and I'm on a mission to ensure it remains:

'th'

There are over four thousand words in the English language which contain the 'th' sound and some of them are among the most common words used.  However, in the last couple of years I have a noticed a deterioration in the pronunciation of it.  Often, children and adults are replacing it with a 'v', 'ff' or 'd' sound.  

Before training to teach, I spent four months in a children's centre on the outskirts of Mexico City. To bring in some money for the centre, I taught English classes to adult beginners in the evenings.  'Th' is a very unusual sound for South American Spanish speakers; they don't have any sounds which require them to push their tongue forward like that.  I always encouraged them to show their tongue to ensure they were making the right sound.  "Quiero ver tu lengua," (I want to see your tongue) became a regular phrase in the English lessons.  It was hard work for them but when they persevered they made much clearer English sounds.  Why, then, are we not encouraging native English speakers to make the real 'th' sound?

When I first started recognising this as a problem among my primary students, I found it hard to correct children because I felt like it might be an inappropriate thing to insist on.  After all, very few people speak the highest quality English and I was worried about highlighting the fact that children weren't speaking correctly.  It was when I realised this lazy form of speaking was having a negative impact on the spelling of some of my best writers that I started taking action.  

Below you can see some of the errors I've come across since September.  These are from a range of abilities and contexts. 

fin - thin

somefing - something 

ve/de - the

dere/fer - there

dat - that

phrone - throne

deaf - death

over - other

To try and tackle this and prevent the correct 'th' sound from being extinct from some children's speech, I've made it explicit in my class that every time we us a 'th' sound, it needs to be correct.  They can correct me, I can correct them and, most importantly, they can correct each other.  To help remind them, I've put this poster up on our Scribble HQ wall.  

It has quite shocked me the extent to which some children struggle to make the 'th' sound but, with the highest of expectations now, I am hoping to prevent extinction and improve the high-frequency spelling of my class.

UPDATE: I have recently added the poster below to my walls after discovering that well over half my class cannot correctly pronounce the number three.