I was recently working on a project and needed to use Ubuntu for a coding segment. I replaced my Windows system with Ubuntu 18.04, but what I actually needed was on Ubuntu 16.04. I have the ISO on an external hard drive and I have been trying to install it in GRUB. To no avail, I have looked across many other forums and none relate to my current situation. Any help would be great. Thanks.
You don't 'install' an ISO in grub. Grub is the Linux boot manager and loader and has nothing to do with ISOs. Its Windows equivalent is the Windows boot manager which is part of the System partition, and you wouldn't install an ISO in there.
What you should do is make a live Ubuntu 16.04 UFD from your downloaded ISO, by extracting the contents from it to the UFD. You can do this easily from 18.04. Then simply boot up from the UFD when it's done. If you have enough space you could make a few extra partitions on your HDD, and install 16.04 from the UFD onto there, or better still a separate HDD. From Grub you will then have the option to boot to 16.04 or 18.04.
Alternatively you can use a virtual machine for installing 16.04. But my experience was that Ubuntu doesn't work so well in a VM, with quite a few gremlins (eg wireless connectivity issues) occurring for no apparent reason.
Reinstalling ubuntu through windows is a solution, but for some, it can be a bit frustrating. For this purpose, we can download your ubuntu 16.04 .iso from the website while you are at ubuntu itself, and install it at ubuntu. This will show a window that you would get while installing ubuntu, but a bit different, You would get a list of options. Choose the option which would be something like "Install ubuntu in place of ubuntu 18.04". Then, click the option 'keep my files'. Then, wait for the installation to complete. Finally, click on the reboot option. Your system will reboot, and then, if all goes well, you should be running ubuntu 16.04 with all your files and compatible apps.