Dear Yong Peng,
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Y.P.: Thanks for pointing out the "vajjabhayadassaavino" is in the
dative.
This way, it agrees with "vajjabhiiruno" in number (singular) and case
(dative). However, the commentary seems to treat both words as plural
nominative:

* vajjabhiirunoti vajjabhiirukaa.
* vajjabhayadassaavinoti vajjaani bhayato dassanasiilaa.
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I repeat the text:

Tasmaatiha, bhikkhave, eva.m sikkhitabba.m�
'di.t.thadhammikassa vajjassa bhaayissaama, samparaayikassa vajjassa
bhaayissaama, vajjabhiiruno bhavissaama vajjabhayadassaavino'ti.
Eva~nhi vo, bhikkhave, sikkhitabba.m. Vajjabhiiruno, bhikkhave,
vajjabhayadassaavino eta.m paa.tika`nkha.m ya.m parimuccissati
sabbavajjehii"ti.
------

The Co: vajjabhiirunoti vajjabhiirukaa.
* vajjabhayadassaavinoti vajjaani bhayato dassanasiilaa.

I think the Co. does not point out the grammar here, but rather the
meaning of siilaa, ways of practice, explained as vajjabhiirukaa,
dassanasiilaa. How else would we explain the ending in bhiirukaa?
Only, siila is neuter, so it should be silani. Or is it a variant?
The Co. elaborates on the noble Persons who attain the four stages of
enlightenment and will be free from all wrongs. Dassana can mean:
seeing by insight. Siila includes many degrees of practice as is also
pointed out by the Co. to this text.

To return to the grammar of the sutta text: I think it can be dative
singular.
Nina.

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