A rule of Shva which modern Hebrew speakers [unfortunately] have forgotten is the following:
However, if it is joined with one of four [guttural] letters, א ח ה ע, its manner [of pronunciation] will be like the manner of the vowel of the second letter in that word, such as: בְּֽהֹנוֹת ידיהם ורגליהם (Jud. 1:7) = bohonoth; מתי פתים תְּֽאֵהֲבוּ פתי (Prov. 1:22) = tei’eihavu; עיניו לְֽחֵלְכָה יצפנו (Ps. 10:8) = leiḥeiləkhah; שריה רְֽעֵלָיָה מרדכי (Ezra 2:2) = reiʻeiloyoh.
Resource — Yemini Hebrew article.
So for instance in Genesis 1:3 it would be wrong to read יְהִ֣י as yə-hî ( check Bible Hub). The Shva here takes the sound of the Chireq .
So same in Genesis 1:14, וְהָי֤וּ should be read wau (as in caught) - hau- yū , not as wə-hā-yū (yikes) :
The Shva in this case borrows the sound of the Qamats from the very next letter.וַיֹּ֣אמֶר אֱלֹהִ֗ים יְהִ֤י מְאֹרֹת֙ בִּרְקִ֣יעַ הַשָּׁמַ֔יִם לְהַבְדִּ֕יל בֵּ֥ין הַיֹּ֖ום וּבֵ֣ין הַלָּ֑יְלָה וְהָי֤וּ לְאֹתֹת֙ וּלְמֹ֣ועֲדִ֔ים וּלְיָמִ֖ים וְשָׁנִֽים
etc..