Can i just switch out the sim cards of my iphone 5c's?

I broke my old iphone 5c, and my girlfriends parents have an extra one. Could i just change out the sim card, and it would work?

Generally speaking, yes, you can just put the SIM into a different phone and it will work. The only time this would be a problem is if one or both of the iPhones use the old CDMA system (Verizon or Sprint), in which case changing the SIM will not work.

Like you said in the comment, your old phone is T-Mobile and the new one is from Sprint which means they're incompatible now. To make it compatible with T-Mobile SIM card, you've to get the phone unlocked so the phone can work on T-Mobile. If you need help with getting the phone factory unlocked, mail me for more info.

Depends on if that other 5c is from the same carrier or unlocked as the one you have now.