The sh’va is a Hebrew vowel, consisting of two dots on top of each other under a Hebrew letter. Sometimes it makes a short “i” sound (“ih”), like the beginning of the word “interesting”. This is called a “sh’va na”. Other times it is silent, effectively closing a syllable. This is called a “sh’va nach”. While some texts will bold the sounded sh’va not all will, so here are some rules for figuring out which is which.
Basic Rules for the Sh’va
1. If the sh’va is under the first letter of a word, it is sounded.
2. If the sh’va is under a letter in the middle or at the end of a word, it is silent.
3. “If two sh’vas go walking, the second does the talking” — if you have two sh’vas in a row, the first one is silent and the second one is sounded.
Intermediate Rules for the Sh’va
1. If there is a dot (called a “dagesh”) in the letter that has a sh’va under it, the sh’va is sounded.
2. If there are two of the same letter in a row and there’s a sh’va under the first one, the sh’va is sounded.
3. If the letter before the sh’va has a trope or meteg (vertical line under a letter that indicates where the emphasis goes when saying the word), the sh’va is sounded.
Advanced Rules for the Sh’va
With appreciation to Cantor Neil Schwartz
Some rules for when a Sh’va is a “sounded” Sh’va Na [ שְׁוָא נָע [ סְ :
1. Under the first letter of a word (exception is שְׁתַּיִם and its derivatives שְׁתֵּי / שְׁתֵּים).
2. Under a consonant with a Dagesh [ סְּ ] (including one that looks like a Dakesh Kal).
3. Following a Long Vowel which does not have a Primary Accent [ ּּקוֹלְךָ = Ko-l’-cha when there’s a trope under the final letter. ]
4. Following a “Secondary Accent” on a Long Vowel [ Meteg עִירְךָ with a meteg under the first letter and a trope under the last letter ] or [ Trope וַיֹּאמְרוּ with a munach under the yud and a zakef-katon over the resh ]. Note: (This can also be said: “A Sh’va between two marked accents is a Sh’va Na”).
5. Under the first of two identical letters [ הִנְנִי ] ( This is similar to the Dagesh Chazak ).
6. The second of two consecutive Sh’va symbols (the first Sh’va is “silent”) [ נַפְשְׁךָ ].
7. Under the consonant following many prefixes [ ,…כַּ…, מִ…, שֶׁ…, בַּ…,לַ… הַ ].
However, after the “Conjunctive Shuruk” [ ...וּ], most scholars say Sh’va is Nach.
Appendix A: Other details about Hebrew syllables
Note that these sheets are under copyright by Cantor Neil Schwartz and are only to be used with proper attribution
Appendix B: Sh’va Meracheif
If you really want to get technical about things….
"Shva meraḥef" is the grammatical designation of a shva which does not comply with all criteria characterizing a shva naʻ (specifically, one marked under a letter following a letter marked with a "short", not a "long", niqqud-variant[↑]), but which does, like a shva na’, supersede a vowel (or a shva na’) that exists in the primary form of a word but not after this word underwent inflection or declension.
The classification of a shva as shva meraḥef is relevant to the application of standard niqqud, e.g.: a בג״ד כפ״ת letter following a letter marked with a shva meraḥef should not be marked with a dagesh qal. The vowel preceding this letter could be represented by the short niqqud-variant for that vowel.[↑] This sometimes, but not always, reflects pronunciation in Modern Hebrew; e.g. מַלְכֵי ('kings of') is commonly pronounced in accordance with the standard form, /malˈχej/ (with no dagesh qal in the letter kaf), whereas כַּלְבֵי ('dogs of'), whose standard pronunciation is /kalˈvej/, is commonly pronounced /kalˈbej/ (as if there were a dagesh qal in the letter bet). In standard syllabification, the letter under which a shva meraḥef is marked is grouped with the preceding syllable.
https://en.wikipedia.org/wiki/Shva