For the sake of illustration, in this answer I shall represent the shva as a colon in the middle of the word (:). There are four rules for determining whether a shva is na (‘moving’) or naḥ (‘resting’):
- If it’s in the first syllable, it is always na — e.g. l:daber, m:saper
- If it’s at the end of a syllable, or word, it is naḥ—e.g. miv:ḥan, itakh:
- If there are two consecutive shvas—the first is na, the second is naḥ—e.g. yish:m:u (pron. yish-mé-u)
- If it’s a single shva in the middle of a word, determine the basic form of the word (masc. sing. for a noun; sing. of the same tense if a verb): if a full vowel appears in the basic form, the shva is na—e.g. y:ladim [basic form: yeled], tivd:qi [masc.: tivdoq], p:qidah [paqid].
Generally speaking, a shva naḥ is a pause, and a shva na is pronounced like a standard /e/—but not always: the word p:qidah, for example, is pronounced pqidah, not peqidah.
So for pronunciation purposes, it is best to forget the four rules above, and just go by the good old IIAMIN rule, which works for most pronunciation questions in all languages: If It’s Awkward, Make It Not:
- If it’s easy to pronounce it as a pause (as at the end of a syllable—e.g. lif:nei, bil:ti ), then do so (a shva naḥ)
- If it’s awkward, then pronounce it like a standard ségol (/e/) sound—e.g. lehitgaber, mitmodedim (a shva na)