My husband misplaced his iPhone and then he left out of town and I found it. I turned it off and sent it overnight to him and it is now lost in the mail. Can I track his iPhone if it is turned off?
You cannot track it if it is not connected in some way to a data network, either over your mobile connection, or WiFi. If it's turned off, or out of reception, you cannot track it.
Depending on what version of the OS is installed, then when the phone is turned on again and gains reception it should start to be traceable, and you can put it in Lost Mode in the Find my iPhone app so that it will alert you and optionally display a message on the device, however you need to have turn this feature on in the device first for this to work.
If the unit is running iOS6, then anyone who eventually finds it will be able to wipe the device and restore a blank OS without having to turn it on thus bypassing this security, but if you are on iOS7 then this is not possible, and the unit is worthless to anyone finding it.