verb
BAD : After just two months he retired and went to work for a smaller company.
GOOD : After just two months he resigned and went to work for a smaller company.
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retire = leave your job at the end of your working life, usually because you have reached a particular age: 'In the UK, men usually retire at the age of 65 and women at 60.' 'If you retire early, you won't get your full pension.'
resign = leave your job because you do not like it, because you have found a better one, etc: 'If she doesn't get a salary increase, she's going to resign.'