She must have forgotten to let you know. 她一定是忘记告诉你了 | Desvl English Tips

She must have forgotten to let you know.
她一定是忘记告诉你了。

He must not have told you that.
他一定是没有跟你说那件事。

must have done something 的否定形式是 must not have done sth
虽然 must have not done sth 也是语法正确的，但这种用法并不自然/不地道