When should you use the imperfect tense and when should you use the perfect tense in French?

The imperfect tense can be used to describe the continuous nature of an action. For example, 'We were playing on the computer'. Additionally, the imperfect tense is used if we want to say an action was repeated or habitual. For instance, to convey this in English, we use the auxiliary verb 'used to' or 'would': 'We used to/would play on the computer'. In French, we would therefore say: 'Nous jouions sur l'ordinateur'
The perfect tense (or the compound past) however, is used to describe an action that begun in the past which continues into the present. For example, 'j'ai perdu les billets' (I have lost the tickets) - the tickets aren't here now which is how it differentiates from the imperfect tense. Moreover, in spoken language, an action that takes place entirely in the past which would normally be in the simple past usually takes the form of the perfect tense. For example, 'la semaine dernière ils ont joué un beau match' (last week they played a fine match). The simple past is rarely used in spoken language in French.