The window was broken by Andy.
Review some verb in plain/past/passive forms. Write the plain form on the blackboard and ask/select volunteers to say all 3 forms out loud. If the student is correct write all 3 on the blackboard and repeat with the whole class. Circle the passive form in pink chalk but don’t say why. This will automatically inform the students that the class with focus on passive without having to needlessly explain the complicated English to the students. Using English that is too difficult to explain something will make the students “switch off”. Keep the flow moving at a good speed and focus on what you will be teaching in the class. Be sure to cover all the verbs that will be used in the class, plus others.