In the sentence below, my grammar checker is telling me to use was instead of were, and I don't understand why. It has a compound subject with a list of three things. I could replace them with they so were seems like the right verb form.
Can you explain?
"For 180 million years during the Triassic, Jurassic, and Cretaceous periods, the land, the seas, and the sky were ruled by dinosaurs and giant reptiles."
— BZ, Thailand
Answer
You are right, and your grammar checker is wrong!
In a sentence with a compound subject - in this case "the land, the seas, and sky" - the verb should be plural. And the clue you used, that this subject can be replaced by were, is a good one to use when checking for verb agreement.
Your grammar checker was probably fooled by the fact the noun right in front of the verb, sky, is a singular noun, and it was unable to see that the subject of your sentence is not just sky.
This is a good example of why we cannot depend on our grammar checkers in our digital devices: They make mistakes!
