In theory, a "patach chataf" - a patach with a shva next to it, like this:
is pronounced in Lithuanian Hebrew like a patach followed by a yud.
This results in the common pronunciations (which I have even heard amongst those who are not Litvish) "Mairiv", "gaiva", and "maiseh".
Is there a rule for how this pronunciation is applied? It seems irregular as I have never heard "rachaimim", "ahaiva", or "chaidasha". How about "Ya'aikov"?