Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
măgistĕro, and, sync., măgistro, āvi, ātum, 1, v. a. [magister], to perform the office of a director or chief, to rule, direct, command: magisterare regere et temperare est, Paul. ex Fest. p. 152 Müll.; cf.: magisterare moderari, id. p. 126: inter manipulares vitam militarem magistrans, Spart. Hadr. 10.