Here is the explanation right out of my new book on word pairs, which will be out this fall. Email me if you want practice exercises.
We don’t need to worry about the word affect, when it is pronounced with the short a. First, there is no problem “hearing” the a and therefore getting the right word. It is the psychiatric term that refers to the outward manifestation of subjective feelings. Similarly, the word affectation is not a problem.
...He had a noticeably flat affect.
...It was clear from his affect that he was depressed.
...It is a really annoying affectation of hers.
The trouble comes because of the words are pronounced with the schwa (uh) sound, “uh/fect,” which makes the words indistinguishable. The dictionary is of little help with affect and effect as both definitions mean influence and affect is listed as “to have an effect on.” There are over ten definitions for each word. So we have to have another way to distinguish these two homophones.
Put aside the psychiatric term mentioned above. We look to the part of speech.
Rule 1: If it is a noun, it is effect.
...The warning had no effect on his behavior.
...I believe it is about cause and effect.
...The effects of the drug will become evident.
Rule 2: If it is a verb or verbal and bring about or make happen can be substituted into the sentence, it is effect.
...The law on cell phone was effected the first of the year.
...The death of his father effected a change in his behavior.
...She wanted to effect a startled reaction from everyone.
This is the problem word. Using bring about or make happen in the needed form is a great way to “test” whether effect is correct. If these just don’t fit, then the answer has to be affect.
Rule 3: If it is a verb or verbal and does NOT mean bring about or make happen, it is affect.
...I will not be affected by this decision either way.
...Her sudden departure will affect Mr. Ross’s status with the company.
...She doesn’t want him to be affected by this.