Final answer:
The sentence 'Cassie was invited to the party by Tim' is written in passive voice because Cassie is being acted upon by the doer, Tim, which can be identified by the use of the be verb 'was' and the preposition 'by'.
Step-by-step explanation:
The sentence "Cassie was invited to the party by Tim" is an example of a passive voice sentence. We can determine this by observing that the subject, Cassie, is not conducting the action, but is being acted upon. The sentence includes the form of the be verb "was," and the past participle "invited." Furthermore, the sentence has the preposition "by" followed by the noun phrase "Tim," indicating that Tim is the doer of the action while Cassie is the recipient of the action.
To convert this sentence to active voice, we would change it to "Tim invited Cassie to the party," making the subject Tim the doer of the action, which is inviting Cassie. In active voice sentences, the subject performs the action, creating a clearer and more direct sentence.