There isn't much different these days to ற and ர - Jaffna Tamil there is a slight difference, but not noticable at all.
"ஞ" and "ந" - the first is pronounced "nya" as in Enya (the singer) but the ny is pronounced together. The second is just a standard n sound.
As a learner you should practice the difference between ழ, ல and ள - as these are very important.