What is the difference between the perfect and imperfect tenses in French and when are they used?

There are mainly two tenses used in French to discuss what happened in the past, these being: the perfect and imperfect.However, both of these tenses are used in different ways based on what the speaker intends to express.
The perfect tense is used to discuss events that have finished and are no longer happening, for example: "La semaine dernière, je suis allé au supermarché." (Last week, I went to the supermarket). We know this is a completed action because it says "La semaine dernière" which indicates that the action was completed last week. It's quite common to see time indications used so that it is distinguishable as to whether it's the perfect or the imperfect tense.
In addition to the usage of the perfect tense, it's important to remember how to form this tense. There are 3 necessary ingredients: the subject/personal pronoun (je, tu, il/elle/on, nous, vous, ils/elles) , the auxiliary verb (avoir or être) in its present tense form (j'ai, tu as, il a...) and the past participle of the main verb (mangé, fini, vécu...). There are some verbs which take the auxiliary être, such as: aller, venir, partir and sortir but these will not be explained in depth here. It's best to remember that most verbs take the auxiliary avoir in the perfect tense.
The imperfect tense is used to describe events that are ongoing and have not been completed, for example: "Je lui téléphonais le jeudi." (I used to call him every Thursday). We know this is an ongoing action because it says "le jeudi" which indicates that it happens repeatedly every Thursday.
Unlike the perfect tense, there is only one verb component to forming the imperfect tense and that is taking the infinitive of the verb, for example: jouer or manger and conjugating it using the imperfect endings, which won't be explained here.