One thing is essential to another thing only if it is of the essence
of it--an important and indispensable part of it, determining its
nature; the soul of it.

_Even_ for _Exact_. "An even dozen."

_Every_ for _Entire_, _Full_. "The president had every confidence in
him."

_Every_ for _Ever_. "Every now and then." This is nonsense: there can
be no such thing as a now and then, nor, of course, a number of now
and thens. Now and then is itself bad enough, reversing as it does the
sequence of things, but it is idiomatic and there is no quarreling
with it. But "every" is here a corruption of ever, meaning repeatedly,
continually.

_Ex_. "Ex-President," "an ex-convict," and the like. Say, former. In
England one may say, Mr. Roosevelt, sometime President; though the
usage is a trifle archaic.

_Example_ for _Problem_. A heritage from the text-books. "An example
in arithmetic." An equally bad word for the same thing is "sum": "Do
the sum," for Solve the problem.

_Excessively_ for _Exceedingly_. "The disease is excessively painful."
"The weather is excessively cold." Anything that is painful at all is
excessively so. Even a slight degree or small amount of what is
disagreeable or injurious is excessive--that is to say, redundant,
superfluous, not required.