Particle Instantiation

Could someone please tell me how I would make A particle system appear while a key is pressed, and disappear when the key is not pressed?

use GetButton and GetButtonUp

That sounds more like GUI. I know that I would use Input.GetAxis() for the keypress.
I just want to know how to start up and turn off a particle system on command.

use something like this

if (Input.GetButton("Fire1")) {
      ParticleEmmiter.emit = true;
}

else if (Input.GetButtonUp("Fire1")) {
      ParticleEmitter.emit = false;
}

Either toggle the properties emit to false or toggle the gameobjects Active status

Maybe the toggling properties of emit would work…

When using the particleEmitter.enabled, it will stop animating already made particles, just leaving them there, halfway through the simulation, until it resumes making more. The gameObject.active trick does not work either…