Depending on what you mean by "lose" your world it could be corrupted or not installed properly and you might have just acedently deleted it but if you did any of those thing I am sorry to tell you but think you will not be able to get it back.
in later versions, if you fly too high and lose your world in Minecraft PE, you can kill yourself by digging to the end of the world. but in 15.1 you can't kill yourself
Yes.
There isn't a way to get your Minecraft PE world from your old phone onto your new one.
There is no way to get to the Nether World, in minecraft PE. The only thing you can do is make a Nether Reactor Core. See related link.
sand cave
minecraft worlds on PE are finite, and the water beyond that is infinite.
there are no mods in minecraft pe i hope there will be
Yes. It is possible to crouch in the minecraft pe.
They may be despawning from your world. This happens in all editions of Minecraft. There is no way to guess when an animal despawns, but they do this most likely so the world isn't crammed with mobs, or to stop lag from occuring.
There are no mods for Minecraft PE But it would be cool
there is no jungle biome in minecraft PE
If you have already purchased Minecraft PE, then update the game.