Choose the correct form of the verbs

1.When i was doing my homework, my friend rang/was ringing.
2.While we walked/were walking to school,we were seeing/saw an accident.
3.The students turned/were turning off the lights when they were leaving/left the classroom.
4.While you were taking/took the dog for a walk,you friend was arriving/arrived.
5.My sister went/was going to university when she finished/was finishing her exams.
6.When i met/was meeting Joe, i was liking/liked him immediately.
И если кому не сложно, то объясните хотя бы вкратце когда что подставляется. Заранее спасибо