Here's why:
* Subject-Verb Agreement: The sentence has two subjects ("Mary Stuart, Queen of Scots" and "was caught") but only one verb ("was caught"). It needs a second verb to correctly connect both subjects.
* Infinitive Phrase: An infinitive phrase is a group of words that includes the word "to" followed by a verb. There is no "to" followed by a verb in this sentence.
Here are a couple of ways to correct the sentence and include an infinitive phrase:
1. "Mary Stuart, Queen of Scots, was caught plotting to overthrow Elizabeth I." This sentence uses a single verb ("was caught") to connect both subjects and includes the infinitive phrase "to overthrow Elizabeth I" to describe the action of plotting.
2. "Mary Stuart, Queen of Scots, was caught and was plotting to overthrow Elizabeth I." This sentence uses two verbs ("was caught" and "was plotting") to correctly connect both subjects. The infinitive phrase "to overthrow Elizabeth I" describes the action of plotting.
Remember, correctly formed sentences require agreement between subjects and verbs, and infinitive phrases can provide valuable context to the action being performed.