In general, when a word is prefixed by Hei HaYdiah (the definite article), the following consonant receives a Dagesh Hazak (gemination). Clear exceptions are when that consonant is a guttural and cannot receive a Dagesh. However, in the Bible in some cases, it seems like omission of the Dagesh occurs even in non-gutturals, specifically the letters Yud, Lamed, Mem, when these are pointed with a Shva. I'm wondering what the rationale for that could be? What is particular about those letters? It doesn't seem like it would be difficult to pronounce them geminated.
(Inspired by this question.)