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In order to convert the definition you were given into a single sentence, you need to identify ways in which the different parts of the definition are related. Use that relationship to determine how to go about constructing your sentence.
In your definition, you are given the general term "monastery" and some description of the components that are included in that object. Specifically, you are told that a monastery is made up of men or women who are called by special titles and who have made certain promises, called vows, when they enter the community.
One possible explanation based on the information you have been given could be, "A monastery is a community of persons, called either monks or nuns, who have taken religious vows upon entering the community."