Can I unplug my external usb drive when not in use?

You can unplug it when not in use without any problem.

There is no harm in only using it when you need it.
Failure rates are measured in power in hours, so keep that in mind. Granted that most disk failure rates occur during spin up or loss of power, in your utilization it is better in the long run for this limited home use application.

Yes, turn it off when your not using it. No need to leave it spinning all the time. Even if it’s smart enough to spin down after some idle time keeping it on does you no good.
Just make sure you “eject” the drive before you turn off the power on it to assure that anything being written out to it is flushed out of your operating system buffers.

Yes.

Just remember to “eject” it so you can disconnect it safely. If you unplug it without “ejecting” it first, you may run into problems, possibly serious ones. But considering how little you use it, you would be better off unplugging it when not in use.