Try this. 1. Turn off your iMac. 2. press and hold the command and r keys, and then turn your iMac back on, but continue to hold down command and r. This should take you into os ten recovery. Don't worry, You aren't going to have to format your harddrive and reinstall os ten from skratch. If asked for a firmware password, enter it and press the enter key. If you don't have a firmware password set, then you can skip this step. At the os ten utilities screen, go to the utilities menu in the menu bar, and click terminal. At the terminal prompt, enter "resetpassword" and press enter. The reset password utility should open in the background. You may have to switch to that window. If you want, you can close the terminal application. You won't need to type anything else. In the reset password window, select the drive or partition os ten is installed on. By default, their should only be 1 listed, called macintosh hd by default. If you have multiple operating systems installed on your iMac, select the disk or partition that contains your forgotten admin password. After that, click on the popup button, and select the name of your admin account in question. If you only have 1 account on your computer, the options will be your own user account and the root account. Do not select root. Select your admin account that you forgot the password for. Now, type in a new password in the password and confirm password fields, and then click save. The utility should Display a dialogue box telling you that the password was reset. Now, you can click the okay button, close the reset password application, and then restart your iMac, and let it boot normally. Note, when you login to your admin account with your new password that you set, you might get a dialogue box about the key chain password. If you remember the old password you used, the one you forgot, click on the update Keychain password button, and type the password you used before you reset it. If you don't remember the old Keychain password, don't worry, just click the create a new key chain password button. Your done, you shouldn't get that dialogue box again. If you don't want to do anything with the Keychain at this time, just click the continue login button. You shouldn't get notified about the Keychain until the next time you login to your admin account. I hope this wasn't too complicated for you to follow, and that your forgotten admin password was successfully reset. I can definitely understand the way you might feel when you try to login to your computer, but you can't because you forgot your admin password. That can be very frustrating and can be a pain too. I hope you have a great night and that this helped you. Please reply too this message if you need any further assistance, or if this tip didn't solve your problem, and I will try to further assist you as best I can. Have a great night.