Hello Duplicine, Welcome to the forum! The essential thing about the passive construction is that the subject of the verb (passively) receives the action, instead of (actively) performing it, and thus corresponds to the object of the active verb. In the sentence "Mary took good care of the children", the object of "took" is "good care" ("the children" is part of the adverbial phrase "of the children"): hence, strictly speaking, the passive should be the rather awkward "Good care was taken of the children by Mary".
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