Our Master coach Daniel calls this ‘The Nightmare Sound’.
Native English speakers won’t understand why, they’ve never had to learn how to pronounce it as adults. All their pronunciation was learnt during their tender childhood age. Because of this, they have no idea how easily and effortlessly they are making the /th/ sound throughout their speech. In fact, they may even think they are actually pronouncing it but they aren’t.
To pronounce an isolated /th/ sound, we lift our tongue to touch our front teeth just enough so that air can pass through. If the air does not pass through, it will sound like /D/.
Now the rest of this text may sound confusing but I highly advise you pay close attention here. Most non-native English speakers over emphasise the /TH/ sound when they are learning how to pronounce it. The smooth sound of words such as ‘the/other/another/something’ contain a halt whenever the person reaches the /th/ sound. Let me give you an example, imagine someone saying ‘other’, now imagine someone saying ‘o-THer’. Now imagine the same with the word ‘some-THing’. The emphasis on the /th/ sound takes over the importance of other sounds in the same word.
To break this habit, you must stop touching your tongue on your teeth completely. Yes, absolutely stop, until the tongue learns not to touch the teeth. For example, try and say the following words without the /th/ sound: another, without, something, this, that, together. They should sound something like: ano-er, wi-out, some-ing, -is, -at, toge-er.
Once this habit is broken, you reintroduce touching your teeth to your tongue. This time, you will do this movement swiftly and gently so that your tongue barely touches your teeth at all but quickly moves towards it and away again. Try this out as many times as you can, using a mirror will help, Bigly. 😉
All the best.