It's easy to pronounce the letter 'r' in "lernen". But when it comes to pronouncing "lernst", it becomes really hard to keep pronouncing 'r' next to "nst". Listening to some native speaker pronunciations, I realized that 'r' is omitted and not pronounced in "lernst", which makes its pronunciation much easier. Is this only a special case where 'r' is omitted, or is there any general pronunciation rule which is also being applied here?