एकादेश उदात्तेनोदात्तः

The single substitute of an unaccented with an उदात्त (acute accent) vowel is उदात्त।
The word "of an anudatta" is understood here. An unaccented vowel, which combined with the preceding udatta vowel remains as a single substitute, becomes udatta. Thus for the udatta इ of अग्नि꣡ and for the case-ending औ which is anudatta, there is always substituted long ई single by 6.1.102. This single substitute will be udatta according to the present sutra, as अग्नी꣡ ॥ Similarly वायू꣡, वृक्षैः꣡, प्लक्षैः꣡ ॥

Why do we say "with udatta vowel"? Observe पचन्ति, यजन्ति ॥ Here पच् + शप् + अन्ति = प꣡च् + अ + अन्ति । Here शप् is anudatta by 3.1.4, so also अन्ति by 6.1.186. The ekadesa of these two non-accented अ will be anudatta. In forming this para-rupa ekadesa by 6.1.97, the svarita of the अ of शप् caused by 8.4.66 is considered as invalid or asiddha.

Other examples are क्व वोऽश्वाः (Rig. V. 61.2) and क्वावरं मरुतः ॥ The word वः is anudatta by 8.1.21. read with 8.1.18. The word अ꣡श्वः is acutely accented on the first, as it is formed by adding क्वन् to अश् (Unadi I.151). The स् of वस् is changed to व् 8.2.66, which is again changed to उ 6.1.113. Thus वोऽश्वः ॥ Here अ becomes purva-rupa by 6.1.109, which is udatta. क्व is formed from किम् by अत् affix 5.3.12 and 7.2.105 and is svarita 6.1.185. The word अ꣡वर is acutely accented on the first by the Phit II.6. The single long substitute is udatta_.