I
verb
BAD : I thought that the long illness would effect my chances of passing the exam.
GOOD : I thought that the long illness would affect my chances of passing the exam.
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affect sth (verb) = have an effect on sth (noun)
Compare: 'There is no doubt that smoking affects people's health.' 'There is no doubt that smoking has an effect on people's health.'
When used as a verb, effect means 'bring about': 'The new president effected several major changes.'
II
noun
BAD : Many women stop smoking during pregnancy because of the effects to the baby.
GOOD : Many women stop smoking during pregnancy because of the effects on the baby.
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effect on sb/sth : 'These rays can have a harmful effect on the skin.'