<p>Forest fires, long (thought to be) a detriment to the environment, (are) now understood (not only) to be (unavoidable) but also to be (a boon) to the forests.
The answer is no error! But I'm confused because unavoidable is an adjective while boon is a noun? Thanks!</p>
<p>Another problem I’m confused about reads: (After having) read numerous diet books, Charles (decided that) the simplest plan would be the best: eat (fewer calories) and (exercise more).
The answer is no error, but can someone explain why exercise more is correct?</p>
<p>It seems you are treating guidelines about parallelism WAY too strictly.</p>
<p>You are correct that “unavoidable” is an adjective while “boon” is a noun. But they are both complements following “to be.” That’s OK. Parallelism problems in real life are usually in-your-face obvious. Example:</p>
<p>Forest fires are now understood not only to be unavoidable but forests also benefit from them.</p>
<p>That’s a real mess.</p>
<p>I don’t understand why you think the exercise sentence contains an error. The stuff that follows the colon works like this:</p>
<p>[verb phrase] and [verb phrase]</p>
<p>The two halves don’t need to match more exactly than that.</p>
<p>So for parallelism the construction on one side of a conjunction doesn’t always have to match the construction on the other side?</p>
<p>In parallelism, the phrases’ headwords must match. In this case, they are both verb phrases, so “to be” matches, producing good parallelism.</p>
<p>@marvin100 Ohh so as long as the words before match, then the construction of the parts after don’t matter?</p>
<p>Not “before” vs. “after” but “headwords.” Phrases have head words. “large, shiny, blue beetle” is a noun phrase, despite beginning with three adjectives, for example. Any noun phrase (although a gerund would be “less parallel”) is going to be parallel, whether or not it has three, two, one, or even no adjectives.</p>
<p>Hmm I thought the error in the first one would be “a boon”. Since forest fires is plural, doesn’t “a boon” have to be too? "forest fires…are boons to the forest? Isn’t this similar to the fact that you can’t say: The students want to become “a doctor”. Multiple students can’t become one doctor. Can someone explain this to me because I make this mistake very often.</p>
<p>Nouns don’t have to agree: The dessert was three pieces of chocolate. Note that this is different from your doctor example, which is illogical because it would require multiple people to somehow be spliced together into one human centipede physician…</p>