I will explain this in detail for the versions of the sound T. The different versions of the sound D will be analogue
For the actual pronunciation please refer to the video that is posted in the second part of this blog post.
For an English native speaker (and for speakers of most non-Indian languages) learning how to pronounce the four different sounds of T will be something that they actually have to practice. Probably in front of a mirror.
The first distinction between the four T sounds that exist in Nepali is between the retroflex T (ट/ठ) and the dental T (त/थ). The terms retroflex and dental refer to the place where the sound is produced (in both cases here with the tip of the tongue).
are all produced by tilting the tongue backwards and upwards, touching the top of the roof of the mouth (the palate)
are produced by touching the edge of the upper teeth with the tip of the tongue:
Both sounds allow for another distinction between aspiration and non-aspiration, which will be discussed in the second part.