निपात एकाजनाङ्

A particle 1.4.56 consisting of a single vowel, with the exception of the particle आङ् is प्रगृह्य।
This sūtra consists of three words: निपातः "particle", एकाच् "consisting of one vowel," अनाङ् "with the exception of आङ्."

All such particles technically called nipäta 1.4.56 which consist of a simple vowel without any consonant, are pragřihya and therefore are not liable to the rules of sandhi. As इ इन्द्र "Oh, Indra" उ उत्तिष्ठ "O arise."

It follows from the above definition that those nipätas which consist of two or more letters, vowel and consonant, are not pragrihya, e. g. प्र in प्राग्नये वाचमीरय.

आङ् which is not a pragrihya, has four significations:- As a diminutive particle when joined to adjectives and nouns, as आ+उष्णम् = ओष्णम् "little hot, tepid" (2) as a prefix to verbs, it expresses the senses of "near" "nearto," and with verbs of "motion," "taking" &c., it shows the reverse of the action, as गम् 'to go', आगम 'to come;' आ+इहि = एहि, (3) as showing the limit inceptive (अभिविधि) "from," "ever since," as आजन्मनः "ever since his birth," (4) as showing the limit exclusive (मर्यादा), "till" "until" as आ+अध्ययनात् = आध्ययनात्, "until the reading begins." When the particle आ has not any one of the above four senses and is used in expressing regret &c., it is a pragrihya, as आ एवं किलासीत् "Ah! such there once was", आ एवं मन्यसे "Oh! do you think so."

The word एकाच् in this aphorism has a peculiar meaning. It means "consisting of one vowel and nothing else." In other words, the vowels themselves when used as निपात, e. g. आ, उ, इ &c. But the term एकाच् has a wider signification than that given to it in this sūtra. It means "a word or a portion of a word consisting of one vowel which can be pronounced at once"- in other words एकाच् means a "syllable". In that sense the words प्र, नि, वि &c., will also be एकाच्, but not so here. Therefore the rule of sandhi will apply to them, thus प्राग्नये वाचमीरय. See sūtra 6.1.1 where the word एकाच् means a monosyllable.

Why do we say "a nipata?" Because if the single vowel is not a particle, but something else, an affix &c., then it will not be a pragrihya, as the final अ in चकार "he did", is the affix of third person perfect tense, and is liable to sandhi; as चकारात्र.