I have a 2008 Ford Focus, and sometimes it starts, and sometimes it doesn't. I have replaced the battery, starter, relay and ignition switch. It happens more in the summer than winter, it doesn't make a sound when you turn the key but the lights and everything on the dash come on. With jumper cables it will start right up. What could the problem be?
Look for corrosion at your battery cables and especially at the starter connection, and all electrical grounds. Clean off the corrosion with a steel brush if you can, replace the ends and lengths of wires if the corrosion is too invasive.