How do I know what kind of past tense to use?

The perfect tense (eg. j'ai fait, j'ai joué), formed through combination of a past participle and a conjugated form of avoir or être, is used for completed decisive actions.

Meanwhile, the imperfect tense (eg. je faisais, elle jouait), formed by adding the endings (ais, ais, ait, ions, iez, aient) to a stem formed by removing -ons from the 'nous' form of the present tense verb, is used for ongoing, continuous or incomplete actions.