pronunciation of ו

[גולם]

In בתשע ורבע and בשמונה וחצי why is the conjunctive particle pronounced with an a-vowel rather than with a e-vowel as it is elsewhere?

 

[Abaye]

"Why" is a tough question. חצי and רבע behave like that, and also some idioms and numerals in certain cases.

השפה העברית - ו' החיבור

 

[utopia]

In biblical Hebrew VAV before a word with only one syllable and words that are stressed in the penultimate syllable (segoli) the stress made this change.

And sometimes the pronounciation simply holds today.

עשרים ושש, כפתור ופרח, הנה והנה

And even the letters of בכ"ל sometimes behave like that: לבטח, לעד, זה בזה, זה לזה, כאלה

 

[Ali Smith]

I believe the rule is as follows: if the conjunction ו is followed by a noun that is stressed on the first syllable (this includes all monosyllabic nouns), the ו gets a קמץ, regardless of what vowel the first syllable of the following word has. Here are some examples:

יום ולילה (day and night)

עץ ואבן (wood and stone)

וְהָאָרֶץ הָיְתָה תֹהוּ וָבֹהוּ וְחֹשֶׁךְ עַל־פְּנֵי תְהוֹם וְרוּחַ אֱלֹהִים מְרַחֶפֶת עַל־פְּנֵי הַמָּיִם
בראשית א ב

וַיֹּאמֶר אֱלֹהִים יְהִי רָקִיעַ בְּתוֹךְ הַמָּיִם וִיהִי מַבְדִּיל בֵּין מַיִם לָמָיִם
בראשית א ו

 

[radagasty]

If the conjunction ו is followed by a noun that is stressed on the first syllable (this includes all monosyllabic nouns), the ו gets a קמץ, regardless of what vowel the first syllable of the following word has.

Hmm... I am not convinced by this proposed rule, because the examples cited, although correct, are exceptional.

In pairs like יוֹם וָלַיְלָה and עֵץ וָאֶבֶן, the conjunction is vocalised וָ because there is a particularly close relationship between the two elements joined together, thus also לֶחֶם וָמַיִם, for instance. This is why it is so vocalised in תֹהוּ וָבֹהוּ.

Otherwise, the vocalisation of ו is regular, thus:
עֲצֵי אַלְמֻגִּים הַרְבֵּה מְאֹד וְאֶבֶן יְקָרָה
לֶחֶם לֹא אָכַלְתִּי וּמַיִם לֹא שָׁתִיתִי
יֹום לְיֹום יַבִּיעַ אֹמֶר וְלַיְלָה לְּלַיְלָה יְחַוֶּה־דָּעַת

 

[Abaye]

I see no reason to guess or invent rules for this, many resources list the rules, one of them appears above.
Specifically for @radagasty's examples, both ואבן and ולילה appear in the bible sometimes with וְ and sometimes with וָ.

 

[radagasty]

Specifically for @radagasty's examples, both ואבן and ולילה appear in the bible sometimes with וְ and sometimes with וָ.

Yes, that's true, but the way you state it suggests that the choice of vocalisation is arbitrary or random. The expected vocalisation for these words is וְאֶבֶן and וְלַיְלָה, and that is how they occur in the bible, unless there is a particular reason for the strong vocalisation, e.g., in the pairs יוֹם וָלַיְלָה and עֵץ וָאֶבֶן, on account of the close relationship between the two items of each pair, viz. day-and-night and wood-and-stone.