Latin verbal system: how perfect and aorist joined in the new perfect?

[Wilmer "Xelloss" Ricciotti]

In latin we have the following perfects, depending on the verb:

- a 'weak' perfect obtained in the most verbs using the -vi/-ui
characteristic (and the first question I have is: where does this
characteristic come from? is it cognate to the *wos/*ws characteristic
we have in greek perfect participles (lelykos, lelykyia, lelykos <
lely-kWOS, lely-kWSja, lely-kWOS))?

- a sigmatic perfect ('scripsi' from 'scribo'), which comes from the old
aorist

- a perfect obtained using the 'doubling' (I don't know if it's the
correct term in english) ('mo-mordi' from 'mordeo', 'de-di' from 'do'),
which is a typical indoeuropean characteristic for the perfect

- perfects obtained by lenghtening a vowel in the stem ('lEgi' from
'lego') or without characteristic ('lui' from 'luo')

- perfects obtained from a different stem ('fui' from 'sum')

What I can't understand is the criterion which was used to choose for
the new pefect an aorist form instead of a perfect one and vice versa.
Where does the -i ending for the 1st pers. sing. in the perfect come
from? In greek we have -a in the perfect as well as in the aorist and in
forms completely cognate to latin ones as 'oida' (< woid-a) is to
'vid-i' (< void-i) or as 'fe-ci' is to'(e)the-ka' ('cappatic' aorist
from tithemi) (here latin conserves even the -k-, and extends it to the
present tense).

Where does the (archaic) -ere ending for the 3rd pers. pl. in the
perfect come from? I've been told that -ere is the old perfect ending,
while -erunt is the old aorist one...

Well I think that's all, at the moment.

Bye
--
Wilmer Ricciotti - Italy
wirix at tin.it