So I heard that if you carry a mortgage and pay interest for your primary residence, you will get some tax money back at the end of the year. However, if you rent it out, it won't bring you any tax benifit.

Then what if you buy a multi-family house and rent out part of it. You still live in the building yet you have someone to "help" you out with part of the mortgage. Can you still claim tax benifit while filing your returns?

This question came to me when I found a place to live for a friend. The landlord still live in the property.

Huh.

To answer my own question: in MA, if your property lease out less than 4 units, and you still live in your property, it does not count.