This is the 11182nd most frequent Turkish word.
dönmüştü
"Had turned" or "had returned."
The word 'dönmüştü' expresses a completed action in the past, related to the movement of coming back after an initial departure.
Kapıdan çıktıktan sonra hemen geri dönmüştü.
After stepping through the door, he had immediately come back.
The word 'dönmüştü' is used here to describe a past action of returning, which occurred as a result of another past event happening (the rain starting).
Yağmur yağmaya başlayınca piknik alanından dönmüştü.
When it started to rain, he had returned from the picnic spot.
The word 'dönmüştü' is used in the past perfect tense, indicating that the action of returning was completed before another past reference point.