noticed where they come up in the processes of inflection etc.; a few require mention here.
136. In internal combination:
a. The augment a makes with the initial vowel of a root the combinations āi, āu, ār (vṛddhi-vowels: 235), instead of e, o, ar (guṇa-vowels), as required by 127; thus, āita (a + ita), āubhnāt (a + ubhnāt), ārdhnot (a + ṛdhnot).
b. The final o of a stem (1203a) becomes av before the suffix ya (original ia: 1210a).
c. The final vowel of a stem is often dropped when a secondary suffix is added (1203a).
d. For the weakening and loss of radical vowels, and for certain insertions, see below, 249 ff., 257–8.
137. In external combination:
a. The final a or ā of a preposition, with initial ṛ of a root, makes ār instead of ar: Thus, ārchati (ā + ṛchati), avārchati (ava + ṛchati), upārṣati (ÇB.: upa + ṛṣati; but A.V. uparṣanti).
b. Instances are occasionally met with a final a or ā being lost entirely before initial e or o: thus, in verb-forms, av’ eṣyāmas AB., up’ eṣatu etc. AV.; in derivatives, as upetavya, upetṛ; in compounds, as daçoni, yathetam, and (permissibly) compounds with oṣṭha (not rare), otu (not quotable), odana, as adharoṣṭha or adharāuṣṭha, tilodana or tilāudana; and even in sentence-combination, as iv’ etayas, açvin’ eva, yath’ ociṣe (all RV.), tv’ eman and tv’ odman B.; and always with the exclamation om or oṁkāra.
c. The form ūh from √vah sometimes makes the heavier or vṛddhi (235) diphthongal combination with a preceding a-vowel; thus, prāuḍhi, akṣāuhiṇī (from pra + ūḍhi, etc.).
138. Certain final vowels, moreover, are uncombinable (pragṛhya), or maintain themselves unchanged before any following vowel. Thus,
a. The vowels ī, ū, and e as dual endings, both of declensional and of conjugational forms. Thus, bandhū āsāte imāu; girī ārohatam.
b. The pronoun amī (nom. pl.: 501); and the Vedic pronominal forms asmé, yuṣmé, tvé (492 a).
c. A final o made by combination of a final a-vowel with the particle u (1122b): thus, atho, mo, no.
d. A final ī of a Vedic locative case from an i-stem (336f).
e. A protracted final vowel (78).
f. The final, or only, vowel of an interjection, as aho, he, ā, i, u.
g. The older language shows occasional exceptions to these rules: thus, a dual ī combined with a following i, as nṛpátī ’va; an a elided after o, as átho ‘si; a locative ī turned into a semivowel, as védy asyā́m.
