so - he (personal pronoun, subject)
kāyapariyantikaṃ* - end-of-body or body ending (adjective)
jīvitapariyantikaṃ* - end-of-life or life ending (adjective)
vedanaṃ - feeling (noun, object)
vedayamāno - experiencing (present participle)
vedayāmi** - I am experiencing (present tense verb)
iti - thus (indeclinable)
pajānāti - knows (present tense verb)
* tappurisa or kammadhāraya compound - both adjectival
** vedayāmī’ti = vedayāmi iti - euphony
A literal translation:
He, experiencing an end-of-body feeling, knows thus, 'I am experiencing an end-of-body feeling.'
He, experiencing an end-of-life feeling, knows thus, 'I am experiencing an end-of-life feeling.'
A more fluid translation:
When experiencing a feeling associated with the ending of the body, he knows 'I am experiencing a feeling associated with the ending of the body.'
When experiencing a feeling associated with the ending of life, he knows 'I am experiencing a feeling associated with the ending of life.'