Yes, that is where I make my D; near the root of my front teeth.
"Near the root of my front teeth" is the wrong place.
"One-inch-backward from the root of my front teeth" is correct.
When you make D, you make
the top of the tongue and
the upper jaw near the root of the front teeth contact each other in a very short time, and then you make them apart, creating the explosive sound.
Here, "upper jaw" means "maxila" or "supramaxilla" as medical terms.
In order to make Japanese R, you have to make the top of the tongue contact to
the posterior supramaxilla, near the uvula.
(
"Near the uvula" is an exaggeration. The distance between the root of your front teeth and your uvula is about two inches, right? So, "one-inch-backward from the root of the front teeth" means
the center of the supramailla.)
Anyway you should try to touch
the supramaxilla as backward as possible with your tonge. This is the knack.
Furthermore, I recommend you to change to use a different tongue part.
When you make D, you make
the upper side just near the top of your tongue contact to the supramaxilla.
When you make Japanese R, however, you should make
the under side near the top of your tongue contact to the central supramaxilla.
In order to do that, your tongue should be
rolled up backwards.
Just try it!
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In case that my English is still difficult for you to understand, I'd explain the knack in a different way.
When you make R in English, your tongue and supramaxilla never contact each other, right?
Instead of that you pull your tongue backwards, right?
Then,
if you pull your tongue backwards and at the same time make your tongue contact to your jaw, you can make Japanese R.
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The difference between L and Japanese R:
L has a longer contact time than Japanese R. Make it shorter to contact.
L has a wider tongue area to contact. Use more smaller area of the tongue.