Q. This sentence has a dual subject but the author has a singular verb, which sounds right to the ear but can’t be correct, right? Here is the sentence: “Building and extending sewer systems requires large capital investments.” Should it read “require” to match the “building and extending” or can those two things be somehow considered as a single thing? Thank you!
A. A compound subject usually takes a plural verb, but not if the components in the subject are being considered as a single unit or concept. The line is subjective and may depend on context:
Kneading and stretching dough isn’t as easy as it looks.
Kneading and stretching dough are two separate but related skills.
The first example considers the two activities together, as a related set of actions; the second considers these same actions separately.
Nor does the choice of verb depend on the additional noun, which has merely been elided before the conjunction. Even if the noun is repeated, the considerations are the same:
Kneading dough and stretching it isn’t as easy as it looks.
And if you reduce the example to the gerunds, either of the following would also work, depending on context and intended meaning:
Kneading and stretching takes practice.
Kneading and stretching take practice.
In your example, “requires” is probably fine, but if the context suggests that the investments may apply to building and extending in separate stages, your safest choice would be “require.” And if there’s any doubt about the author’s intention, choose the plural.