The subject-verb agreement rule which is being broken in the sentence above is c<span>ollective subjects functioning as a unit require singular verbs.
The collective subject <em>the crew </em>requires a singular verb, because it denotes a single unit. So, instead of using the verb <em>have visited, </em>it should have been <em>has visited. </em></span>
It says in your reading, so for someone to answer that we need to know the reading material.
The two subjects of the sentence are Colonel Lloyd and his "finely cultivated garden".
