1
be unfastened
表示状态，用be动词+过去分词作表语。
The belt was unfastened.
腰带被解开了。
解开（尤指衣服或带子）；松开
先理解核心含义，再看它在真实句子里的用法。
解开（尤指衣服或带子）；松开
to cause something to become undone; to become undone or untied
The belt was unfastened.
腰带被解开了。
unfasten
Base
原形
unfastens
3rd person
第三人称单数
unfastening
-ing
现在分词
unfastened
Past
过去式
unfastened
Past participle
过去分词
I unfasten my seatbelt.
我解开安全带。
She unfastened her seatbelt as soon as the plane landed.
飞机一着陆，她就解开了安全带。
He unfastened the buttons on his shirt because it was too hot.
因为太热了，他解开了衬衫的扣子。
The child unfastened the straps of her backpack to take out a book.
孩子解开了背包的带子，拿出一本书。
表示状态，用be动词+过去分词作表语。
The belt was unfastened.
腰带被解开了。
过去式形式为 unfastened。
She unfastened her necklace.
她解开了项链。
unfastened 作定语修饰名词。
He noticed the unfastened button.
他注意到了那颗未扣好的纽扣。
Not this
Please unfastened your coat.
Use this
Please unfasten your coat.
错误使用了过去分词 unfastened 作为祈使句的动词原形，应该用动词原形 unfasten。
Not this
The window is unfastening.
Use this
The window is unfastened.
描述窗户的状态（未关紧）应使用过去分词 unfastened 作形容词，而不是现在分词 unfastening，后者表示正在进行的动作。
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Focus on the distinctions below. Other candidates are kept compact until a reliable usage note is available.
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动词：解开（尤指衣服或带子），松开；形容词：未系紧的，未扣好的
Dictionary facts from ECDICT, WIKTIONARY.