REDUPLICATION

Look at the first syllable of the root:

  1. Take the first syllable of the root and the first consonant and first vowel.
  2. Change aspirated to unaspirated
    kha → ka   gha → ga
    cha → ca    jha → ja
    ṭha → ṭa    ḍha → ḍa
    tha → ta    dha → da
    pha → pa    bha → ba
  3. Change gutterals to palatals
    k→c    kha→cha   ga→ja   gha-→jha   ṇa→ña
  4. If the root starts with a sibilant followed by a stop, reduplicate the stop
    ś ṣ s stops are the first 25 vowels
    stu→tu sthā→ta ścut→cu skand→ca
  5. If the vowel is long, shorten it
  6. If radical vowel is e→i, o    or au →u
    sev→si    dhauk→du
  7. Roots that end it e,ai, o are treated as if ending in ā
    gai→jā
  8. Vocallic ṛ and R are represented as i.
  9. h turns into j
 From yesvedanta.com.au
Rule: Example:
1. Long vowel  >  Short vowel
(ā > a, ī > i, ū > u)		 √bhī  भी  3P (to fear)            >   bibheti      बिभेति       * Class 3
√dā    दा  3U (to give)           >   dadāti       ददाति        * Class 3
√dhā  दा  3U (to place)        >   dadhāti     दधाति        * Class 3
2. Aspirated  >  Unaspirated
(kha > ka, gha > ga, cha > ca, jha > ja, ṭha > ṭa, ḍha > ḍa, tha > ta, dha > da, pha > pa, bha > ba)		 √bhī    भी    3P (to fear)         >   bibheti       बिभेति       * Class 3
√bhas भस्  3P (to blame)     >   babhasti    बभस्ति      * Class 3
√dhā   धा    3U (to place)      >   dadhāti      दधाति       * Class 3
3. ṛ ऋ   >   i इ √bhṛ भृ  3U (to carry)            >   bibharti     बिभर्ति       * Class 3
√ghṛ  घृ 3P (to shine)            >   jigharti       जिघर्ति       * Class 3
√pṛ    पृ  3P (to protect)        >   piparti        पिपर्ति       * Class 3
4. h ह्   >   j ज् √hā   हा 3P (to abandon)     >    jahāti         जहाति       * Class 3
√hu   हु  3P (to sacrifice)      >    juhoti         जुहोति       * Class 3
5. If root starts with 2 consonants,
only 1st consonant reduplicates.		 √jval    ज्वल्  1P (to blaze)          >   jajvala         जज्वल      * Perfect 
√prach प्रछ्  6P  (to ask)             >   papracha    पप्रछ        * Perfect     
√tya     jत्यज् 1P (to abandon)    >   tatyajitha    तत्यजिथ   * Perfect
6. If s स् is followed by consonant,
remove s स् and consonant reduplicates.		 √stip   स्तिप्  1Ā  (to drip)        >    tiṣṭepa        तिष्टेप         * Perfect
√stuc  स्तुच्   1Ā (to shine)      >    tuśṭoca        तुश्टोच      * Perfect
√svap  स्वप्   2P (to sleep)      >    suṣvapitha  सुष्वपिथ    * Perfect         


√sthā  स्था    1P (stand)           >    tiṣṭhati         तिष्ठति      * Present    

      • Notice reduplication even applies to few roots for Present tense.
      • √sthā is irregular, meaning it'll have slightly different rules outside what you'd expect. The irregularity is that last ā आ reduplicates as i इ and not a अ as per rule 1. Hence it's: ti  ति.
      • Why does s स् become ṣ ष् ? Because if s स् is preceded by anything, except a/ā अ/आ – it retroflexes. See VOL2PG346.
      • Also notice how retroflexed ṣ ष् further affects the following t त्, changing it into retroflex ṭ ट्.
      • Another rule is mentioned on VOL2PG72#6. If root starts with s स्, and that s स् is followed by a stopsparśa  (which is usually t त् or p प्) — then only the stop (t त् or p प्) is repeated.
7. Velars (guttural) convert into palatals.            

Velar/guttural:
ka क,  kha ख,  ga ग,  gha घ,  ṅa ङ

Palatal:
ca च,  cha छ,  ja ज,  jha झ,  ña ञ

 √ghṛ     घृ    3P (to shine)      >   jigharti    जिघर्ति     * Class 3         


√khād  खाद् 1P  (to eat)        >  cakhāda  चखाद      * Perfect
√ki  कि   3P (to perceive)      >  ciketi        चिकेति    * Class 3    

 

√kṛ    कृ  8U (do)                    >  cakāra      चकार      * Perfect
* Irregular. Hence ṛ ऋ doesn't change into i इ per rule 3.
8. Samprasarana Rule:
(Seen in Garden #30, Corresponding semi-vowel)             
  • If ya य is first letter of root, replace with: i इ.
  • If va व is first letter of root, replace with: u उ.
 √yaj   यज्  1Ā       >   iyajitha    इयजिथ      * Perfect
√vac  वच्  2P        >  uvāca       उवाच        * Perfect
Other rules applied sometimes on some roots:
1. Vowel + first consonant of root are repeated. √dā    दा  3U (to give)           >   dadāti       ददाति        * Class 3
2. First syllable (usually) appears in weaker form. √dā    दा  3U (to give)           >   dadāti       ददाति        * Class 3
3. The vowel that's vṛddhi/guṇa grade will usually reduce in strength. √dā    दा  3U (to give)           >   dadāti       ददाति        * Class 3