Rule 1: 1. Setting the scene with the habitual present
Modern stories often open in the imperfect, describing what the hero does every day: يَبْدَأُ يَوْمُ مَنْصُورٍ كُلَّ صَبَاحٍ …. The same habit in the past is expressed by كَانَ + imperfect (كَانَ مَنْصُورٌ يَعْرِفُ زَبَائِنَهُ). The event that breaks the routine is then told in the perfect, often introduced by a time phrase such as وَفِي صَبَاحِ أَحَدِ الْأَيَّامِ 'one morning'.
يَشْرَبُ مَنْصُورٌ قَهْوَتَهُ كُلَّ صَبَاحٍ فِي الْمَقْهَى.
yashrabu manṣūrun qahwatahu kulla ṣabāḥin fi l-maqhā
Every morning Manṣūr drinks his coffee in the café.
Habit in the present.
كَانَتْ تَأْتِي كُلَّ يَوْمِ خَمِيسٍ.
kānat taʾtī kulla yawmi khamīsin
She used to come every Thursday.
Habit in the past.
وَفِي صَبَاحِ أَحَدِ الْأَيَّامِ وَصَلَتْ رِسَالَةٌ غَرِيبَةٌ.
wa-fī ṣabāḥi aḥadi l-ayyāmi waṣalat risālatun gharībatun
One morning a strange letter arrived.
The event: perfect.