I have an hdd on my old computer i wanna put it on my new one i know how to install it inside but i wanna know if there is a problem if the old hdd has windows 7 installed on it and the current hdd has windows 10.
You will not be able to run the applications from the Win7 installation.
Assuming you do not accidentally boot from the old drive (if it appears to try to, hit reset, and jump into BIOS and select your Win10 drive as the first boot device in boot device priority menu.
You may need to take ownership to access the files on the old WIn7 drive, but, should be able to browse all files/folders. or, recover/transfer any files needed. When finished, it can be quick formatted and used a secondary drive within WIn10.