i bought a guitar from my cousin a week ago and today my dad tells me that after every time you play it you have to loosen the strings because if you don`t the neck of the guitar will start to bend. is this true?

Whoa - where`s the science? A guitar should be tuned normally to A440 pitch, which is what they are engineered for. That will match the piano, and I would think be compatible with all other instruments, even most songs on the radio etc. When the guitar is tuned to the pitch for which is is designed, it can stay there. You do not have to slacken the strings. But beware, heat will tighten the strings, sometimes the heat from your hand, but you don`t have to worry about that unless you want to be in perfect tune with something else. An upward change in temperature or humidity will cause the guitar neck to move and stretch the strings, which you don`t want. Then, you can slacken the strings. It goes without saying - own a tuner and use it. Check the guitar often. Don`t leave it near a source of heat, and use other common sense modalities in storing it when not playing. It is common for necks to require some seasonal adjustment, or adjustment if a different guage of strings are used. See any neck adjustment or truss rod topics for info on that. Strat Man Dwight & RockItSciencelabs - Advanced Guitar Technology

Wow I worded that real wrong I meant to say; After you use your guitar and you`re going to put it back in its case do you have to loosen the strings?

I don`t loosen my guitar strings when I put it away, for up to a month in the case. I have other guitars to play then. Yours is fine if you have it in tune, not too tight, like the guy says, away from the heat and whatever.