येन विधिस्तदन्तस्य

This sutra consists of three words :- येन 'by what (attribute),' विधिः 'rule,' तदन्तस्य 'having that (attribute) at its end.'

This is a rule of interpretation. When a rule is made with regard to a particular attribute or letter, it means also words having those attributes or letters at their end. Thus there is a rule 3.1.97 declaring \let there be the affix यत् after the vowels.\ Here the phrase \after the vowels\ means and includes \after the roots ending in vowels, as well as roots consisting of a single vowel.\ Thus चि + यत् = चेयम्. Therefore, this sutra means that when a rule is laid down in this grammar with regard to a particular attribute, that rule, besides being applicable to that particular attribute, is also applicable to words ending in that particular attribute. Thus a rule laid down generally with regard to vowels, will apply to words ending in vowels; a rule laid down with regard to certain forms, will apply to words ending in those forms.

Thus the sutra एरच् 3.3.56 'after इ there is अच् 'declares that the affix ach would come not only after इ, but after any expression that ends in इ. Thus इ + अच् = अयः, चि + अच् = चयः ॥

Vart:- The present rule, (called tadanta-vidhi) must not be applied to rules relating to compounds (samasa), and to pratyayas (affixes). Thus sutra 2.1.24, declares that a word in the accusative case is compounded with the words srita, atita &c. As कष्टं + श्रितः = कष्टश्रितः 'involved in pain.' The present sutra must not be applied here, and we cannot say that a word in the accusative case would be compounded with another that ends in srita; for tadanta-vidhi does not apply here. Hence we cannot form a compound of कष्टं + परमश्रितः ॥

Similarly, in rules of affixes. Thus sutra 4.1.99 declares \that the words नड &c., take the affix फक्.\ Thus नड+ फक् = नाडायनः । But we cannot apply the affix phak to the word सूत्रनड, the tadanta-vidhi not being applicable here. The descendant of सूत्रनड will be called सोत्रनाडि and not as above.

The above vartika is however qualified by the following:

Vart:- The above vartika does not apply to words formed by affixes that have an indicatory उक् (उ, ॠ, लृ), short or long, or where a rule is propounded with regard to letters. Thus the affix ktavatu 1.1.26 has an indicatory उ, and we have कृतवत्. A rule which will apply to kritavat will apply also to the word which ends in kritavat. Thus the feminine of kritavat is कृतवती 4.1.6, the feminine of sukritavat will be sukritavati. Similarly rule (IV. I. 95) अत इञ् says \after अ there is the affix इञ्.\ This is a rule relating to pratyayas, but as it is propounded by mere letters, the rule of tadanta-vidhi will apply here. Thus दक्ष + इञ् = दाक्षि 'the son of Daksha.'

Vart:- When a term, which denotes a letter, is exhibited in a rule in the form of the locative case and qualifies something else which likewise stands in the locative case, that which is qualified by it must be regarded as beginning with the letter which is denoted by the term in question, and not as ending with it.

Thus sutra 6.4.77, achi snu dhatu bhruvam yvor iyan uvanau, means \iyan and uvan are the substitutes of what ends with the pratyaya snu and what ends in a verbal root in इ or उ, whether long or short, and of the inflective base bhru, when an affix beginning with a vowel follows.\ Here the word 'achi' is exhibited in the seventh case, which literally means 'when a vowel follows;' but by virtue of the present vartika it means, 'when an affix beginning with a vowel follows,' as, श्रियः, भ्रुवः ॥,