In Standard English, ‘th’ is pronounced and formed with the tip of the tongue touching the top row of teeth. It is known as a voiceless or voiced dental fricative (IPA θ or ð). However, in many places, there is a noticeable tendency for this sound to change.
– USA (New York City, Chicago, and Philadelphia), this sound becomes a dentalized ‘d‘ or ‘t‘ sound.
– London, the voiced th often becomes ‘d‘ at the beginning of a word: this becomes ‘dis.’ Meanwhile voiceless th becomes ‘f;’ mouth therefore is pronounced ‘mouf.’
– Dublin, th simply becomes plain old ‘t‘ and ‘d:’ ‘ting,’ ‘dis,’ etc.*
– Singapore, th is changed to ‘t‘ and ‘d’.
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The Dester advert is very clever to use the mispronounced ‘th’ as a catch phrase.
Why is this? The standard English ‘th’ (θ or ð) exists only in few languages, many people who speak English as a second language use alternate pronunciations.
For Chinese speakers in Singapore there is no ‘th’ sound in any of the dialects. The same is true for the Irish language.
Interestingly another word that trips up some Irish and Singaporean people is ‘film’. It is mispronounced as ‘fil-im’ when it should be said along the lines of ‘fill-mm’.