Effect:
noun
2: something that inevitably follows an antecedent (as a cause or agent)
Affect:
transitive verb
1: to produce an effect upon: as
a : to produce a material influence upon or alteration in <paralysis affected his limbs>
b : to act upon (as a person or a person's mind or feelings) so as to produce a response : influence
unlike then and than, there is a very acute similarity between affect and effect. However, this similarity does not arise due to the two being synonyms, but rather because affect is the act of causing an effect. To put it more simply, effect is most commonly used when describing a noun meaning "something has been influenced". In contrast, affect is the word used to describe the verb meaning "something has produced an effect."
In general terms, my rule of thumb is to treat effect as a noun and affect as a verb. Although both of these words have alternate meanings which make them both nouns and verbs, the most common usages of these words follow this form. When you're debating on which word to use, think about whether the sentence needs a verb or a noun at that specific point:
The accident had a severe effect on his speech
OR
The accident had severely affected his speech
With these simple rules, the use of these two words will become much clearer.