How to make skills activate under certain conditions?

My question: how can I make a skill available only if a condition (or two) are met?

Example:

A thief typically has some sort of backstab/sneak attack ability.  Okay, so I want to have a Thief class who can Backstab if he has used another skill, like "Vanish" which makes him Hide (as per the RTP skill list).  But I don't know how to do this, or to even go about doing this.  Can anyone help? [rhetorical question, as I know someone can.]

Thanks, all. :)

EDIT:

Furthermore, how can I design a class so that, once it levels up, certain parameters increase?  For instance, I'd like for this same thief to become harder to hit at higher levels, but I don't know how to insert that into the class "schematics."

You can use Yanfly's skill restriction script for this: http://yanflychannel.wordpress.com/rmvxa/gameplay-scripts/skill-restrictions/

1) Have the first skill give the thief a "hidden" state - and in that state, you add the skill as an effect of the state. Then the skill only becomes available if the actor is in that state.


2) If you want to increas features at later times, you'll need a skript like the "dynamic features" skript

Andar, how do I add the skill as an effect of the state?  I've tried "Learn Skill" but that didn't seem to work.

not learn skill - the feature is "add skill".


However, to use a skill the actor needs both the skill and the skill type - if the actor already has the skill type (usually that is the case), then the add skill is enough. If not, you need to add both skill and skill type

Thanks!  I think I've figured it out...for now...