conjunctions - Verb agreement with a phrase set off by "but"

A coworker is writing a sentence like

Sally, but especially Joe, enjoys questions about grammar and usage.

He thinks it should be

Sally, but especially Joe, enjoy questions about grammar and usage.

Leaving aside the possibility of rewriting the sentence, I want to understand which part of the subject controls the verb's number. Consider the following:

1. 
   

   My parents, but especially my wife, [is/are] supportive of my goals.

   
   




2. 
   

   My wife, but especially my parents, [is/are] supportive of my goals.

   
   





3. 
   

   Dogs, but especially cats, [is/are] the cause of many allergies.

   
   

To my ear, are seems right in all three cases. If that's right, the rule would seem to be that if either part is plural, use the plural verb. I'm least confident about sentence two.

In any case, I'd like to learn more about what's going on here, grammatically. Is the phrase set off by commas a compound subject? An appositive? A subordinate clause?

Answer

In all of your examples, the "but..." clause is parenthetical. That means it should be able to be dropped without changing the meaning of the sentence.

So the first sentence;

"Sally, but especially Joe, enjoys questions about grammar and usage."

can be reduced to

"Sally enjoys questions about grammar and usage.".

There's only one Sally so she "enjoys".

Similarly,

"My parents, but especially my wife, [is/are] supportive of my goals."

can be reduced to

"My parents are supportive of my goals".

You use "are" because "parents" is plural.

Part of the problem with these example sentences is the (mis)use of the word "especially". This is normally used to single out a particular member of the previously referred-to group.

E.g. "The team, but especially Bob, love to drink beer"

which means that they all like beer but Bob especially likes beer.

However, in your examples the subjects of the "especially" clause are not part of the previous group. For example, "Dogs, but especially cats" doesn't work because cats are not a subset of dogs. You could change "especially" to "also", in all of these examples, to give the effect you intended.

Dogs, but also cats, are the cause of many allergies.