nadī

1. The rule

Feminine long-ī stems (type nadī) are one of the small set of Sanskrit paradigms where the stem vowel itself mutates into a consonant under inflection: ī → y before any vowel ending. This automatic glide gives the paradigm its distinctive rhythm. These stems are restricted to the feminine gender; nadī (“river”), devī (“goddess”), nārī (“woman”), rājñī (“queen”) all decline the same way (Whitney §364; MacDonell §100).

2. How to remember

Three features mark this paradigm out from every other vowel-stem:

1. i Nom. pl. ≠ Acc. pl. nadyaḥ (nom.) vs. nadīḥ (acc.). Feminine ī- and ū-stems are the only nominal classes in Sanskrit where these two plurals are distinct. Everywhere else, nom pl and acc pl fall together.
2. ii The ī → y glide. Whenever a vowel ending attaches, the stem vowel becomes a consonant: nad-y-ā (ins.), nad-y-ai (dat.), nad-y-āḥ (abl/gen), nad-y-ām (loc.), nad-y-au (N/A du.), nad-y-oḥ (G/L du.), nad-y-aḥ (nom pl.). Before consonant endings, ī stays long: nadī-bhyām, nadī-bhiḥ, nadī-nām, nadī-ṣu.
3. iii The vocative drops the macron. Voc. sg. is nadi! — short i, not ī. It's the only form in the whole paradigm where the stem vowel shortens. Used as direct address: “O river!”

3. Exercise — nadī

Fill in the full paradigm. The -y- glide cells are the ones to watch: they all have y between nad and the vowel ending.