Q. With a compound subject, does the verb number change when the conjunction “and” is replaced by “and then”? For example: “Swimming in the ocean and then running a marathon require/requires great endurance.” I’m told CMOS 5.138 applies and the verb should be plural (“require”). But it seems to me “and then” has combined the two actions into a sequence (as one) which would take the singular “requires.”
A. Two subjects joined by and can sometimes be considered singular. The test is whether the subjects express a single idea or more than one. In your example, what requires endurance is the combined action of swimming in the ocean and running a marathon—a continuous feat of athletic activity. The adverb “then” makes this clear.
But adding “then” won’t always make a plural compound subject singular. Consider the following sentence, in which the subjects clearly take a plural verb: “A bandage and then an ice pack were placed on the wound.” On the other hand, you can write a sentence with a compound-but-singular subject without the help of “then.” For example, “Peanut butter and jelly is the best thing to happen to sandwiches since sliced bread.”
So it’s best to consider such sentences on a case-by-case basis.