Effective
From Hull AWE
There is sometimes confusion over the spelling of this word.
The adjective that most students are likely to want is effective. It is similar in meaning (though not exactly the same as) to efficient, which might help you to remember the appropriate spelling.
There is a rarely used affective, which means 'to do with the affections'. Modern students are unlikely to want it.
You may also want to consult Affect - effect.