There are no stupid questions, there are only people too stupid to ask a question.
I just leave my car set to EV and so far I have used 0.04 gallons of gas in 2,000 miles.
The computer runs the ICE (ice = internal combustion engine) for a few seconds every week or two to keep the ICE lubricated. When the battery runs low (it keeps two bars in reserve) it automatically switches to ICE power.
The owner manual says to with to HV at high speeds but I don't think it is necessary and have not been give one reason why you can't just leave it in EV all the time. I am basing my opinion on driving 2,000 miles in my Honda and 3 years in a Chevy Volt