That which is enjoined to come in the room of what follows is to be understood as coming in the room only of the first letter thereof.,
When any operation is to be made in a word following another, such operation is to be made in the initial letter of such subsequent word. To give an illustration:- there is a rule 6.3.97 by which it is declared: \In the place of अप् used after the words द्वि and अन्तर् and the particles called उपसर्ग there is ई.\ This rule may be stated in other words as:- 'In the place of अप् there is ई when अप् follows द्वि or अन्तर्.' Now it is clear that ई is not to replace all the letters of अप् but only one. By sutra 1.1.52 that letter would have been the last letter of अप्, but this sutra makes an exception, namely, where an operation is directed to be made in a word, simply by reason of its being placed after another word; such change is to be made in the beginning of such second word. Therefore, the ई replaces the अ of अप् and we have द्वि + अप् = द्वीपं (the final अ is added by 5.4.74) 'an island,' आन्तरीपं 'promontory.' So also in the sutra ईदासः 7.2.83 'long ई is the substitute of अन when the latter comes after the root आस्.' Thus आस् + अन = आस् + ईन = आसीनः. Here also the ई replaces the initial अ of अन.
How can we find when an operation is to be performed on a subsequent word? When the previous word is exhibited in the 5th or ablative case, see sutra 1.1.67.,