Pronouncing -th "incorrectly"
The -th sound in English is one that can be very difficult for non-native speakers to pronounce. I myself have spent a great deal of time with my tongue under my teeth in class trying to teach students how to pronounce it. But here's an interesting question to consider, just how necessary is it pronounce the -the sound with one's tongue under their teeth?
Jennifer Jenkins, a linguist and leading figure in the study of English as a lingua franca, has published a great deal of work focusing on English communication between non-native speakers. In her work, she has suggested that there are some sounds that non-native speakers do not need to reproduce exactly like native speakers since even with slightly varied pronunciation they will still be understood. The -th sound is one such example, which, Jenkins states, can be substituted with other sounds without affecting intelligibility. This is good news for English speakers who struggle with the -th sound as it means that they can substitute it with an easier sound and still be understood.
Of course, if someone who wants to speak with a standard American or British accent (which I don't think is necessary but it's a valid goal nonetheless), they will have to pronounce the -th sound as it is in those accents. What's interesting to note, however, is that there are many native English varieties that do not pronounce the -th sound. In Cockney English, a variety of English from London, the -th sound is pronounced as an f or a v, as in fink (think) or muvah (mother), respectively. In Jamaican English, the -th sound is often pronounced as a t, as in tink (think).
The bottom line is this: if you can't pronounce the -th sound with your tongue under your teeth replace it with another sound and don't worry, people will be able to understand you just the same!
Check out this video to see how the -th sound is pronounced in Cockney English: