The day i failed geometry, crashed into the garage door, and broke my arm was not the best day of my life. this sentence is an example of

The answer is understatement.

We would normally expect the person to finish the sentence by saying it was "the worst day of my life", after all the bad things described. Instead, the speaker chooses to say it in a softer way, to make it sound less abrupt or straightforward, but still counting on the listener to understand the real meaning behind it.

A synonym for understatement is euphemism.

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