I have an unhealthy interest in etymology, although it does have its uses. One of them is checking the right form of a word that's often mis-spelt (or misspelled. Either is acceptable). There are other ways, and some of these mnemonics might help.
The ultimate root is the Latin finis, meaning end. The obvious English word is finish, but the more useful ones are define and finite. When we define something, we show its limits, i.e. where it finishes (and where everything else starts). Something with no end or boundary is infinite. When you draw something: you give it an outline, which is where it finishes. Put another way, you define it. So it's de-finite. With an i.
I've got a set of knives. The small one, for cutting things, is called a paring knife. When you pare things, you cut them. You sePARate them.
Wrong: Moses lead the Israelites
Right: Moses led the Israelites
I can't offer an etymological way out of this one. The past tense and passive participle of to lead is led. The best I can offer, other than an image of me standing behind your desk holding a big stick with a nail in it, is to think of Robert Plant and Jimmy Page, and ask yourself, "Who led Led Zeppelin?"
The letter 'c' is soft when followed by 'e', 'i' or 'y'. That's why the first 'c' in accident is hard (because it isn't followed by 'e', 'i' or 'y') and the second one is soft, so it's pronounced aksident. That's also why you've been mis-pronouncing flaccid all this time. If there were two 'c's in necessary, you'd have to pronounce it neksessary.
Wrong: ecstacy, extasy
Just take out the 'cst'. Then it's easy.
Moral: We all need a few tricks.