Will, the natives here gave excellent answers. As a learner, I would add that once I figured out they are pronounced virtually the same, it mattered a lot less. As a westerner, I needed to put some time in to learn the 'flipped R', which appears in many languages but not english. It is like the hard double d in Ladder, you flap the tip of the tongue against the alveolar ridge. The D is only slightly removed from this, with the toungue moving down the back of your top teeth. The main point is when spoken they are almost indistinguishable.

Din and Rin has the same meaning and function which means in english "also or too" the only difference is that how are you going to use them. The word "rin" is used after a word which ends with a vowel and "din" is being used after a word which ends with a consonant. Sya rin ay mabait - She is also good. Ikaw din ay mabait. - You are also good This is not really a strict rule you can interchange them and no one will notice you used it wrongly and don't be shocked if native speakers sometimes interchanged these two words.

Nothing. ;) You see /d/ and /r/ were once allophones in Tagalog phonology. The same thing happened with "dito/rito", "dine/rine", "doon,roon"... Preference vary in different Tagalog regions. No particular grammar rule applies, so don't worry. Good luck! ;)

no changes! it's exactly the same :)

The change of a word that begins with the letter d to the letter r also applies to single words. For example, dami means plent, a lot, etc. But when used as an adjective the da changes to r, thus marami. Same is true for dumi meaning dirty. The adjective is marumi.