The difference between these two words is a very simple one. They are the past tenses of two different verbs.
‘Bought’ is the past tense of ‘buy’: I bought a new car last week.
‘Brought’ is the past tense of ‘bring’: I brought him a glass of water.
The difference can be remembered easily too, as ‘bring’ shares its first two letters with ‘brought’ (‘br’).
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