ನನ್ನದು is pure Kannada. ನಂದು is colloquial Kannada. ನಿನ್ನದು is pure Kannada. ನಿಂದು is colloquial Kannada.
ये gets translated to ಇದು or ಈ depending on context. A rule of thumb is that we usually shouldn't have two words ending with ದು in the same sentence. [My interpretation: ಇದು is a noun, ಈ is an adjective.] [A Kannadiga friend gave me a counterexample: "this is mine" translates to "ಇದು ನನ್ನದು", which has two words ending with ದು.]
Here we used ಈ instead of ಇದು because ಯಾರದು/ನನ್ನದು/ಅವರದು already ends in ದು.
Note that in Hindi, "वो pen मेरा है।" and "वो मेरा pen है।" are both correct, but in Kannada, "ಆ pen ನಂದು." is the only correct option. [A Kannadiga friend says this is incorrect. "वो pen मेरा है।" translates to "ಆ pen ನನ್ನದು.", and "वो मेरा pen है।" becomes "ಅದು ನನ್ನ pen.".]