I've been using 1.7.10 for modding for a while and am just switching to 1.9 for dual wielding and other new features. But I don't understand most of the code or the "model" system, and I'm also having issues with the textures. If anyone can help with the basics of 1.9 like models, textures, items, blocks, tile entities, multi-sided block textures, connected textures and custom block models would all be helpful topics to get started with. Thanks.

I'm in the same boat.

For simple blocks and items, wuppy29 has a good, simple json generator. The models changed slightly in 1.9, though, so where you see "builtin" you have to change it to "item" or "block" depending on if it is an item or block. For multi-texture blocks, TheGreyGhost has a working example called "Minecraft by Example." mbe_01 is a simple, multi-textured block. Here is the link: https://github.com/TheGreyGhost/MinecraftByExample. As for custom models, I got the OBJ Loader semi-working (it doesn't support some commands yet, which is screwing with my textures), but I don't know much about anything else. Forge has a debug class which can give you the general idea: https://github.com/MinecraftForge/MinecraftForge/blob/master/src/test/java/net/minecraftforge/debug/ModelLoaderRegistryDebug.java and assets: https://github.com/MinecraftForge/MinecraftForge/tree/master/src/test/resources/assets/forgedebugmodelloaderregistry

I have not yet updated my tile entities, so I do not know how to update it. A tutorial from 1.8 might give you some pointers.