When do you pronounce the gutural r and when the twisted one?
I've always had this question. I think it depends on the word but I want to make sure.

Question

Answer

Don't let the spelling confuse you.
Most often, the R is vocalized. Neither uvular nor trilled.

http://www.uni-bielefeld.de/lili/studium/faecher/daf/personen/richter_julia/lehre/ausspracheschulung/fehlerbeispiele/segmentalia/r-laut-V.html

Answer

Unlike in Spanish, in German there's no difference between those r's. For example, I cannot pronounce the trilled ("twisted") r at all, I always use the uvular ("guttural") r. Which would be a problem if I tried to speak Spanish. :-)

In Southern Germany, people are more likely to use the trilled r; in Northern Germany, the uvular r. Use whatever you prefer.