Help me to better grasp ～ていた and ～た - FMHY

For durative verbs this is quite easy for me to grasp: 鳩が見る: I will see a dove 鳩が見ている: I am seeing a a dove 鳩が見た: I saw a dove 鳩が見ていた: I have been seeing a dove But for perfective verbs — it is quite hard for me: 杪冬の前に雪が溶ける: Before the end of winter the snow will melt 杪冬の前に雪が溶けている: Before the end of winter the snow will already be molten 杪冬の前に雪が溶けた: Before the end of winter the snow melted 杪冬の前に雪が溶けていた: Before the end of winter the snow had already been molten In both of the last cases before winter began the snow is already in a state of being molten as a result of the melting being complete. So I often make mistakes differentiating the last two cases and treating them as the same even though I consciously know the ～た emphasizes the action being completed and the ～ていた emphasizes the state change being already completed. Maybe someone can help?