Many writers confuse these two words. The common mistake is to use effect when you should use affect, typically when using it as a verb. Effect can be used as a verb, but its meaning is restricted, and is synonymous with produce:
She effected a change in the way the IRS conducted its audits.
For the sense of to have an effect upon, use affect:
She affected the IRS's procedures.
WRONG RIGHT
A legacy of foreign rule significantly effects a nation's political culture.
A legacy of foreign rule significantly affects a nation's political culture.
She effected a change in the way the IRS conducted its audits.
For the sense of to have an effect upon, use affect:
She affected the IRS's procedures.
WRONG RIGHT
A legacy of foreign rule significantly effects a nation's political culture.
A legacy of foreign rule significantly affects a nation's political culture.