How do I know when to use the ‘imparfait’ (imperfect) and ‘passé composé’ tenses?

Be assured that this question makes all French learners stop and think sometimes! So I will do my best to explain it in a way that helps me remember when to use which tense. Both of these tenses are used to refer to an action that happened in the past. However, they are both used for slightly different past actions. 1) The ‘imparfait’ is used to describe a background of ongoing events, something that used to happen habitually or a past state of emotion and being. Example: Quand j’étais jeune, je n’aimais pas étudier le français, mais maintenant le français est ma langue préférée. = When I was (IMPERFECT) young, I didn’t like (IMPERFECT) studying French, but now French is (PRESENT) my favourite language. Explanation: Because being young is a state of being which lasts a long amount of time, this must be in the imperfect. You didn’t enjoy studying French during this long amount of time, so the imperfect must be used in both cases here as you are describing an ongoing event in the past. When you then say but NOW, the tense changes to the present as you are talking about what has changed in the present situation. 2) The ‘passé composé’ tense is used to describe a one-off event that happened at a specific time. This is a completed action. Example: Hier soir, j’ai regardé un épisode de ‘Love Island,’ puis j’ai dormi. = Yesterday evening, I watched an episode of ‘Love Island,’ (PASSÉ COMPOSÉ) then I slept (PASSÉ COMPOSÉ). Explanation : Because watching Love Island and sleeping both specifically happened last night, the ‘passé composé’ tense must be used here as they weren’t ongoing in the past.3) The ‘imparfait’ and ‘passé composé’ tense can both be used together when a background action was happening, then something else suddenly happens.. Example: Je prenais mon petit-déjeuner, quand le téléphone a sonné. = I was eating my breakfast (IMPERFECT) when the telephone rang (PASSÉ COMPOSÉ). Explanation : In this example, eating your breakfast is the background action. Then, the telephone suddenly begins to ring, so this must be in the ‘passé composé’ as it is a one-off action.