One of the fundamentals of English grammar is that a verb agrees in number with its subject. When we think of a verb, we probably think first of tense, the time when the action is occurring. But verbs also have number, meaning singular and plural. If the subject is singular, the verb should be singular; if plural, plural. Both tense and number are two (of five) properties of a verb, and determining the number of the verb can be risky at times.
Here’s a short passage that is describing the character of someone: He loved the outdoors, all of it: bugs and rocks and dogs and water. But what he loved most were the trees. The first sentence gives us the context for the second, and it is the verb were in the second sentence that directly concerns us here. Were is a plural form of the verb to be in the past tense (was, by contrast, is a singular form), and the question before us is whether were should be was: But what he loved most was the trees.
When we are in doubt about the construction of a sentence, it is always good procedure to begin by separating the subject from its predicate; this will put the subject term into some relief so that we can examine it more easily. Thus: But what he loved most | were the trees. The number of the subject determines the number of the verb, and the subject here, what he loved most, is a bit involved. Read it again closely and you will see that what he loved most means that which he loved most. The word what in such a construction is called a double relative; it stands both for the demonstrative pronoun that and for the relative pronoun which. We recognize which he loved most as a restrictive relative clause whose antecedent is the pronoun that, and those two elements, pronoun and relative clause, together comprise the subject of the sentence.
Now we’re in a position to see that the pronoun that is singular (its plural form is those), and so we have the evidence we need to conclude that the verb for this subject phrase should be singular, not plural: But what he loved most was the trees. So what might be behind the writer’s mistake? The passage is filled with plural nouns: bugs and rocks and dogs, even the outdoors itself is a plural noun in singular construction (which is why, by the way, its pronoun, it, is singular), and so the writer’s cast of mind, so to speak, is in a plural world. But even more determinative is the fact that the noun in the predicate of the sentence, trees, is plural. The proximity of this plural noun to the verb will influence many a good writer, particularly so amidst so many other plural nouns in the context, with a complicated singular subject phrase to boot. But the subject is singular, no matter how involved, and so its verb should remain singular as well.
Such a lapse falls under a grammatical concept called, oddly, attraction; the number of the verb (were) has been attracted by the number of the noun in the predicate (trees), and the attraction proved fatal. Not all attraction, grammatical or otherwise, is so consequential, but when the subject and verb do not agree in number, the match can be misleading. The worry is not about grammatical etiquette but of accuracy, of clarity. If the body and bulk of what we read and write is clear and sure, then we are better able to appreciate the intentional lapses that a good writer will undertake in literature and poetry to help us see what we commonly see in an uncommon way. And then the attraction isn’t fatal but playful, as language full of life should be.