The Japanese "r" is really more like a cross between r, l, and d. The 5 letters it is used in are:
Hiragana: ら、り、る、れ、ろ
Katakana: ラ、リ、ル、レ、ロ
Romanized: ra, ri, ru, re, ro
The sound is made by touching your tongue to the top of your mouth. I don't think mouth shape has any effect on it.
Why do I feel I just wasted 2 minutes of my life posting here?
