I bought an iPhone 7 that I could have been unlocked from iCloud and I signed in my iCloud account and now I forget my passcode if I erase the phone will it go back to the old iCloud or the last iCloud signed in on the phone?

It will be activation locked and will show the Apple ID of the person that last set the iPhone up. Unless you are that person, all you will be able to do to remove the activation lock is to obtain the needed password from them. If you are the one that last set it up, then use:

See If you forgot your Apple ID password https://support.apple.com/en-us/HT201487

You also can use Having trouble signing in/iForgot https://iforgot.apple.com/password/verify/appleid 

And you undoubtedly will need to see If you forgot the answers to your Apple ID security questions https://support.apple.com/en-us/HT201485

Before trying to answer your security questions is very important that you think about your questions and their answers carefully and try to recall all details you can. If you don't answer them correctly enough, then you easily can lose control of your account.