Learn English – Difference between “dawn” and “realize”

I think the difference is in degree. It's similar to the difference between large/huge/gigantic or small/tiny/miniscule. In some contexts, you may be able to substitute any of those words, but due to common usage or word origin or whatever other factors, there is an understood subtle scale.

So, if you have relevant facts to discuss, they may or not be important, but they are related to the matter at hand. But if you have pertinent facts to discuss, they have precise or logical relevance to the discussion. They absolutely should not be overlooked. Pertinent facts are always relevant, but relevant facts are not necessarily pertinent.

One reason why this may be the case is that pertinent could also be used as a synonym for apt (ie. strikingly appropriate) but relevant doesn't really have that meaning. If I said you wrote an "apt answer" to this question how would you feel? What if I said it was a "pertinent answer?" Or "relevant answer?"

I think most people would feel that calling it a relevant answer is nearly a mild insult. Oh yeah, it's related, but not close to perfect.

A voluntary action is one that the actor, employing free will, chooses to take.

An involuntary action is one that takes place out of the control of the actor, or against the actor's will.

Spontaneous has several related definitions. A spontaneous action can be one that the actor takes without a significant amount of forethought ("a spontaneous purchase"). In that case, the spontaneous action is voluntary.

On the other hand, spontaneous can also refer to something that occurs with no apparent cause ("spontaneous combustion"). By its nature, this activity rarely involves an overt "actor" per se. When it involves a living organism ("spontaneous growth", "spontaneous motion", etc.), the action is probably involuntary.

Spontaneous has a few other definitions as well. Generally, you'll have to discern the difference from context. When you see the word "spontaneous" used to describe an action taken by a clearly definable actor, though, the action is probably voluntary (although it might not be).