Hi Bruno
Your subject (plurals in PIE, Latin and Romance langs) is vast, and the
little information you offer is at best only partially correct.
You say:
> in Latin, the desinence for ablative feminine plural is -is;
This is, alas, simply not true, put like that. The -is ending does not
belong to the gender, but to the declension. First declensions may be fem
or masc, and second declension may be fem, masc or neuter; while third,
fourth, and fifth declension words, many of which are feminine, have a
different ending.
There is no morpheme for "feminine" in Latin, although the ending -a on
words which show a form in -us is most likely to indicate a female (but
there are exceptions!!)
PIE (at least at its later stages) was indeed like this, with no clear
morpheme to mark feminines. The ending -h2e might originally have indicated
collectives, and was associated with adjective forms in -h2e whdcih later
develop into what we call "feminine". But it would be anachronistic to use
later labels for an earlier stage - the details are too complex to fit the
simple label.
You also say:
>2) In French, the plural is marked by an -s, but only in writing; only the
>article distinguishes between "le théâtre" and "les théâtres".
This again is only partly true. The plural marker in French is complex. It
can - in speech - be found at the beginning of words which start with a
vowel (/arbre/ and /zarbre/ in l'arbre and les arbres) and is often marked
in other ways, such as liaison with or without -s- after a past participle.
In Italian there are nouns which have no special form for the plural, so the
article carries the weight of indicating the number. All monosyllables,
nouns ending in a consonant, an accented vowel, or unaccented -i are of this
kind, for example il re and i re, la citta` and le citta`, la crisi and le
crisi. But remember that plurality will be indicated by the verb as well
(which is only partly true in French).
Hope that helps more than confuses - there clearly is a lot for you to
research!
Peter