A nasal vowel is ALWAYS substituted for ā before ru when it is followed by a letter of aṭ pratyahāra (i.e. when it is followed by a vowel or ha , ya , va or ra).,
Thus 8.3.9 teaches रु substitution of म, the long आ preceding it, would have been optionally nasal by the last sutra. The present sutra makes it necessarily so. Thus महाँ असि (Rig. III. 46. 2) महाँ इन्द्रो य ओजसा (Rig. VIII. 6. I). देवाँ अच्छादीव्यत् ॥
Some (i. e. the Taittariyas) read it as anusvara. This is a Vedic diversity.
Why do we say \for a long आ\? Observe ये वा वनस्पतीरनु ॥ Why do we say \when a vowel or ह, य, व, or र follows\? Observe भवांश्चरति, भवांश्छादयति ॥
The word नित्य 'always' is employed for the sake of distinctness only. The very fact of making a separate sutra, would give it a compulsory force, even without the word nitya.,