There are two types of Sh'va, they both looks the same but pronounced different.
Sh'va Nach (settled): doesn't have any movement. like the p in the word "laptop".
Sh'va Na (moving): has an "e" sound, same as Segol/Tseire vowels.
there are 2 main rules to recognize Shva Na (they don't always work, there are exceptions):
1) in the beginning of the word (like the first Yud in the word Yerushalaim (Jerusalem) יְרוּשָלַיִם)
2) if there is sequence of 2 following Sh'vas, the first one in Nach and second on is Na (for example in the word Yilmedu יִלְמְדוּ (they will study) the Lamed is shva nach and the Mem is shva na