untie [ʌnˈtaɪ]

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vt.解开，松开；解放

to unfasten a knot or something tied

It's just impossible to untie the knot. It's too tight. 这个结根本解不开。太紧了。
Will you please untie the knot for me? 请你替我解开这个结头，好吗？