Another Agreement Lesson

What’s wrong with this sentence?

The isotopic signature of material from the inner and outer solar system differ significantly.

“Differ” is plural, right? And the phrase “inner and outer” describes two things, right? So what’s wrong?

“Inner and outer” isn’t the subject of “differ”! “Signature” is, and “signature” is singular.

Since we’re talking about two things, though, the subject should be plural! (“Inner and outer” are objects of the preposition “of.”)

The picture isn’t very useful, but it’s in the article…

Isotope analysis indicates that Earth's water may have arrived when the Moon was formed

Remember: the subject and the verb must both be singular or both plural. That’s agreement.