When do you roll your R's?
Like when you say purple in spanish you roll the R
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It's easy to know when you have to roll the 'r' and when not. The 'RR' sound (or erre fuerte) is used when you see a double 'RR', when you have a single 'R' at the beginning of a word or after the letters 'N' or 'L'.
Also, I wouldn't really call the soft R sound as 'rolling'. It has a very slight vibration, but it's just a single touch of the palate with the tongue, pushing a lot less air through the mouth. The real rolling R sound happens in the cases I mentioned above. You hold the tip of the tongue against the palate, close to the teeth, and you push more air through the mouth, making the tongue vibrate a lot stronger.
Rs are always rolled. A single R is usually rolled just once, although at the end of a word it is sometimes rolled more than just once. A double RR is rolled a few times. The amount of times is left to the discretion of the speaker for emphasis.
Once you start speaking you won't even notice that you are rolling them. You just will. And purple doesn't roll maybe just a bit though. Morado. Try arreglar if you want to hear one that rolls.
The preferred terminology is "single/multi tap 'r'" (occasionally "flap" instead of "tap"). In the prevailing usage, "rolling"/"trilling" refer only to the multi-tap sound. Both sounds are, of course, quite distinct from the "retroflex 'r' of English. Japanese, for example, uses the single-tap 'r' but has nothing like the multi-tap 'r'.