unfasten|unfastened|unfastening

你能帮我把这个项链解开吗？

Can you help me ___unfasten this necklace?

他解开安全带下了车。

He ___unfastened his seatbelt and got out of the car.

她解开鞋带准备换鞋。

She is ___unfastening her shoelaces to change shoes.