My original iMac keyboard got damaged, so I bought one from Staples that is compatible with both Macs and PCs (I'm told most are these days). It works fine, except I an't use keystrokes for, say, saving (Ctrl S), copying (Ctrl C), etc. I can perform the functions using the drop down menu, but it's cumbersome. Does anyone know how I can utilize these functions? Any advice is appreciated.

You should be able to "remap" the keyboard:

https://www.lifewire.com/what-...lents-to-mac-2260203

Typically, if you're using a "Windows" keyboard with a Mac, the key equivalents are:
Control = CTRL
Option = Alt
Command = Windows

Mostly the answer is move your thumb over one key. (hit the windows key instead of where the command should be)

I'm in the middle of this myself, using a "pc" keyboard at the moment as I killed my last Mac keyboard. My play was to buy a "mac layout" keyboard in a week or two, but I'm starting to get used to the PC layout and may not.

(my real problem isn't at home, it's on the PC at work. Before they were different enough that I kept the shortcuts straight. Now not as much)

I've always had to remap my windows keyboard to match the Mac equivalent as best as possible.