This is a rather quick one, and I know I’m probably missing something obvious, but any help from you real grammarians out there would help my day out a lot.
In this sentence:
“Not only my parents but also my grandfather want/wants me to be a lawyer.”
Is the correct version “want” or “wants”? I’m instinctively leaning toward ‘want’, but for the life of me I can’t conjure up a rule to explain why.
Does anyone have a source for a rule regarding this?
Thanks.
Just a stab in the dark, but I’d say, “NOBODY wants you to be a lawyer.”
Kidding.
I would just cheat and say:
“Not only my parents but also my grandfather would like me to be a lawyer.”
lol, thanks for the stab, but I’m obliged in this case to provide a comprehensive ruling on this sentence for an ESL friend. I’ve been searching for about half an hour without luck, but I think it may be because I don’t really know what this kind of rule would be called.
For me, it seems like the sentence is saying “Both my parents and my grandfather…”, but I’m not even 100% sure on whether that’s a correct interpretation.
I’m tempted to believe that either want/wants could be correct. In the case of “neither of them” both ‘is’ and ‘are’ are acceptable follow-ups, after all.
It’s “want” because “parents” comes first and that’s plural.
As it is, the phrasing is clunky, so I’d probably go with the “both my parents and grandfather” version, since that’s saying the same thing.
I’d explain it more thoroughly, but I currently have some sort of plague and am not entirely with it right now.
In a compound subject, the quasi-logical inference of the units of the subject is what governs the verb’s number.
“Both John and Mary” (two singular individuals taken together) “have” (not “has”) “4.0 averages. Therefore, either John or Mary is” (not are, they’re singular individuals taken separately) "the proper choice for valedictorian, with the other being salutatorian. Instead, the school board ruled that they are " (not is, they’re being grouped again) “co-valedictorians, with no salutatorian.”
In your sample sentence, the “Not only…but also” construction is the grammatical equivalent of an “and” – a more emphatic one. “Not only apples but also oranges…” effectively says “Apples would be amazing enough, but both apples, and, more amazingly, oranges as well…” Hence, the construction is equivalent to “both my parents and my grandfather as well want me to become a lawyer.” – And the plural “want” is called for because the logical compound subject is plural in form.
Compare the reversal, killing off your father to avoid the plural “parents”:
“Neither my mother nor my grandfather wants me to become a lawyer.” Here we have a statement about the wishes of two singular individuals, joined because the content of each wish is the same, but taken separately. “My mother doesn’t want me to be a lawyer, and neither does my grandfather.” Toss your father back in, and “Neither my parents nor my grandfather want…” becomes appropriate, because part of the compund is plural.
As SharkB8 notes, however, sometimes the best thing to do with a compound subject is recast the sentence to use a verb form where number does not matter.
Ah, there we go. I knew I was halfway there, but couldn’t remember whether “but” acted like “and” or like “neither/nor” and kept the two parts separate.
More tea and honey for me. I wish we had whiskey. 
If you have doubts about something like an ambiguous noun/noun phrase and want to know replace the subject with potato or potatoes (long story).
My potatoes want me to be a lawyer.
I’ve found 99% of the time you can reduce the phrase into a single object (also the “not only” is really clunky so you have to remove things like that).
But the best bet here would be to recast
“Both my parents and my grandfather want me to be a lawyer.”
“Not only do my parents want me to be a lawyer, but my grandfather does too!” for somehting with a bit more oomph to it.
No source, sorry. I vote “want” because the subject seems to be “they.” The typical pitfall is to use the ear and therefore the nearest noun (grandfather, in this case) as the subject, which is what makes some people lean toward “wants.”
IMO the whole sentence is convoluted and so poorly constructed. It should more simply phrased, such as “My parents and my grandfather want me to be a lawyer”, so the tense required is quite clear.