'08 impreza turns over but wont start. Replaced battery twice, new. This morning same thing, cranks and feels like it's going to turn over but doesn't. Attached jumper cables, starts in a heartbeat. Drove it for a few minutes, no problem starting after that. This has happened the previous 4 mornings. It's my son's car and he's driving me crazy.
This sounds like you have a power leak somewhere draining the battery as it sits overnight. This could be a simple as a map light being left on or a trunk light or something similar staying on due to a faulty switch.
The other cause of a power leak would be a short in a wire or component that isn't bad enough to blow its fuse but enough to drain the battery. I've had that happen with a stereo once, it could be many other things though.
Tracing the problem, first I would wait until dark and look for lights still on. Try pushing the glove compartment light switch in to see if it turns off, have someone sit in the trunk when you close the lid to see if the trunk light goes off as well.
Checking the lights takes minutes and no tools, confirming you have a battery leak and tracing the leak to the source requires a multimeter. You connect it inline with the battery's negative battery terminal and look for amps above 50Ma, a serious drain would be well above that. You have to wait for everything to turn off in a modern car though, it often takes up ti 30 minutes for all the electronics to turn off. I'm not going to go into more detail on this because there are several sites to help, for instance this one.