Imagine you're taking an editing test and you come across the following sentence. "Barbara says Jack also is enrolling in classes in libiral arts, algebra, economics and is considering joining the soccer team."
What do you do?
You fix the spelling of "liberal," obviously. Even if your eyes glossed over that, you catch it when you run spell-check. You pause at "says," perhaps wondering if that informal yet popular way of conveying "said" is appropriate for the context.
You consider other places for the word "also," perhaps putting it after the verb "is." You question whether it's a good idea to have the word "in" appear two times so close together. You decide to leave it as is.
Your first fix is right. As for the "says" and the "also," you're right if you change them and right if you leave them. In both cases, either choice is valid. You made a good call with the word "in." It's grammatical, even if it is a little awkward.
You take a second look at your sentence and decide you're happy with your choices. See anything else?
According to someone who recently sent me a test with a similar question, most copy editors miss the other error. See it now? It's in the last part of the sentence, "is enrolling in classes in libiral arts, algebra, economics and is considering joining the soccer team."
In case you still don't see it, here's one final hint: It could be fixed by inserting the word "and" before "economics."
This sentence contains a classic example of an error known as a faulty parallel. This mistake, a very common one among the professional writers I edit, means a phrase that should have been in parallel structure somehow fell short.
Consider the sentence: "Christine likes kale, Brussels sprouts and spinach." What we're really saying is: "Christine likes kale, Christine likes Brussels sprouts, and Christine likes spinach." But we didn't want to repeat "Christine likes" for every item in the list.
Instead, we make the three vegetables share a single "Christine likes." That's parallel structure. The three parallel items are almost like attachments: They can all be tacked right on to the end of "Christine likes."
In English, lists of two or more items take "and" before the last one, even if there were commas between all the others: kale, Brussels sprouts and spinach. The "and" is what gets most people into trouble.
When a sentence gets a bit more complicated, writers can lose track of what that "and" is supposed to be doing — introducing a list item that attaches to a stem the same way all the items before it did. So after the "and" they shift form, say by putting a verb there instead of a noun.
With "Jack is enrolling in classes in liberal arts, algebra, economics and is considering joining the soccer team," we're technically saying that Jack is enrolling in "classes in is considering." We create an implicit combo of "in is" because our stem — the thing all the items in the list are supposed to share — is "classes in."
For an easy fix, you can often toss in another "and": "Jack is enrolling in classes in liberal arts, algebra AND economics, and is considering …" Another option is to make it two sentences, ending the first after "economics" then starting the next with a new noun or pronoun: "He is considering."
Faulty parallels take other forms, too. The phrase "as well as" is a common culprit: "Kent is taking classes in history, trigonometry, anthropology, as well as chemistry." But this, too, can be fixed by throwing in the word "and": "Kent is taking classes in history, trigonometry and anthropology, as well as chemistry."