Learn Turkish: Tam kımıldayacakken rüyasından dolayı tedirgin oldu. - Just as she was about to stir, she became uneasy because of her dream.

It combines the future tense suffix -acak (going to) with the adverbial suffix -ken (while/when). Literally meaning "while she was going to stir," the structure -acakken is the standard way to express being on the verge of an action. Placing tam (just/exactly) before it creates the full meaning of "just as she was about to."

Why does rüya (dream) take the -dan ending here?

The postposition dolayı (because of / due to) always requires the noun before it to be in the ablative case (-dan / -den). The full breakdown of the word is rüya (dream) + -sı (her) + -n- (buffer letter) + -dan (ablative).

Could I say rüyası yüzünden instead of rüyasından dolayı?

Yes! Both mean "because of her dream." Yüzünden is specifically used for negative causes or when assigning blame, which fits perfectly here since the dream made her uneasy. Just remember that yüzünden requires a possessive structure (rüyası), while dolayı requires the ablative case (rüyasından).

What is the difference between kımıldamak and the more common verb hareket etmek?