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 dove
鳩が見た: I saw a dove
鳩が見ていた: I had 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?
I mean, in terms of nuance and meaning, you’ve pretty much got them figured out. Your translations correspond with the way I understand those forms as well, so yeah, 〜た/〜る emphasises the certainty of an act, while 〜ていた/〜ている emphasises the state. If you’re struggling to remember when to use which or you think you mix them up sometimes, maybe you need to practice more listening and reading, and repeating along. That will help you sort of internalise what each means and how it’s used.