A quick overview of how car insurance works in MA. The insurance is with the car and you assign a primary driver to the car on policy, You are allowed to lend your car to anyone you want and they are covered when borrowing your car even though they are not on the policy. However the caveat to that is anyone living in the same household as you must listed on the policy as a secondary driver to be covered. This is under the assumption that people living together with likely borrow each others car more often than anyone not living with the car's owner.

If the dad has the car policy and neither of you is listed on policy you could get away with that if anything happened by your dad telling the agency he lent the car to whichever one of you for that day. But if you wanted to do it right, he should change the "garaging" location to your address, list your partner on the policy as the primary driver and then you as secondary. Note that the insurance cost may change do to change of address and the records and experience of the primary driver. So if she is a relatively new driver or your town is considered higher crime than dad's, these could drive up the premium.