I'll Trick You Into Saying RRRR!
I once tricked a "Yankee" into speaking Southern (for a second anyway) by pointing to the wall and asking him what he called that part of a room that runs between the floor and ceiling..., then what bird is typically served for Thanksgiving... then I told him to say the two words together quickly... the result is the name of a famous whiskey popular in the South!
Well, the point is, people can be convinced that there is just no way they can pronounce some sounds in a foreign language. Sometimes they even believe it is anatomical which, barring genuine defects, is ridiculous.True, they have not ever produced certain sounds and so they can be tough, but there is usually a way.
The first thing you have to point out is that when learning to pronounce the trilled R of Spanish, they have to accept that the written symbol R in Spanish does not have the same sound value in Spanish. That's easy and obvious enough, but they need to realize that their habit of reading R and saying the American English R instead of the Spanish sound value for R is really a psychological barrier.
Next, write the word palmetto on the board and ask the person to say it. Assure them it's an English word. Then tell them that you sort of lied... that the word palmetto is really just the English spelling of a Spanish word that the English speakers heard and wrote it so as to correspond to the sound values of English letters. The original Spanish word is palmera.
The lesson of this of course is to point out that DD or TT in most English words corresponds to the sound value of the single, intervocalic sound value of the letter R in Spanish. Now write palmetta -- and remind them to pronounce the A as in Ah (not Aw).
Ask them to say the word again, and to notice where their tongue is when they say the TT. Then ask them to say Ralph and compare the two positions. When saying the R of Ralph, the tongue is in a position it should never assume in Spanish.
Point out next that the sound they just made is not the trilled R but rather the sound of R when it falls between vowels -- it is called simple or tap R.
Next, have them practice palmetta a couple of times until they see that they are successfully saying palmera. Point to palmera and have them say it. If they revert to the American English R it is a perfect time to prove that the problem is psychological...
Finally, write el rey and gran rey on the board. Tell them that the position of the tongue for the R in these words is the same as the one they just learned for palmera/palmetta but that this time, you are going to insert a little ghost of a letter after the words el and un and before the word rey. Write a letter D just above above and between the two words and ask them to pick one of them and say it.
Frequently, this trick works quite well the first time! Or, if it doesn't, having them say one or both of them several times in a row will somehow make the trilled R seem to "kick in." I've only had it fail once or twice over the years -- really.
Of course, the next task is for them to consistently apply this new ability. That often takes some time, but if they are impressed enough with the fact that the really did make this new sound, then they can be reminded of that success and encouraged to repeat it in other contexts. They also need to learn that the R of all words beginning with an R are trilled, as well as all digraphs -- rr -- found between vowels, such as in perro or arroz. Tell them it's perfectly ok to pretend there is a D just before the --rr-- and that this little set of verbal trainer wheels is acceptable.
Above all, keep a good sense of humor so they relax.