Perfect Tense

past - happened once

e.g. yesterday I went to the park

hier, je suis allé au parc

subject + auxilliary + past participle

il/elle + a/est + verb

if the auxilliary is être then the past participle needs to 'agree' with the subject.

that doesn't happen if the auxilliary is avoir. 

verbs that take the être:

MRS

-monter

-rester

-sortir

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