I call it my 99% rule.
99% of the time
effect is a noun and
affect is a verb (have an effect on, influence).
Only one in a hundred cases where the question is "Should I use
affect or
effect?" requires more than the 99% rule to solve the problem.
And yes,
effect can be used as a verb (produce, create, cause), and
affect
can be used as a noun (feeling, emotion), but these usages are
comparatively much less frequent. For all practical purposes
these meanings can usually be dismissed from consideration.
CJ