R vs. RR
In Spanish, r has a different sound depending on where it is in a word.
Rolled, as in marron(beginning of word).
Tap roof of mouth with tongue, as in water(middle or end of word).
Double r (rr) always has a rolled sound.
You can practice rr by placing the tip of your tongue on the roof of your mouth and exhaling forcefully across the surface of your tongue. As you do so, release the tip of your tongue so that it lightly flutters against the roof of your mouth. It takes some practice, but you’ll get there!