Why 'Left' is Used Instead of 'Leave' in "They Left Japan Last Month"

'Left' is used in the past tense to indicate that the action of leaving Japan has already been completed. 'Leave' is the present tense and would be used in a sentence like 'They are planning to leave Japan next month.'

You could change the sentence to 'They have left Japan last month,' but this construction is not as common in English.