- Bad spelling and grammar is a major turn-off.
- Bad spelling and grammar are a major turn-off.
- Bad spelling and grammar are major turn-offs.
Which of the sentences above is correct?
It depends on whether you take a descriptive or prescriptive view of grammar. A descriptive grammarian would likely say all three are acceptable because you will hear all three.
A prescriptive grammarian like me says that # 2 is bad grammar. There is no question that # 3 is perfectly grammatical: subject, verb, and copula are all plural. The one that raises a grammatical question is # 1.
At least in U.S. English, certain phrases such as bacon and eggs are treated in some contexts as a singular concept despite the plural form. Someone may say Bacon and eggs is my favorite breakfast meaning that the combination of bacon and eggs is what I like best for breakfast. In that example, it is the combination of foods that is being referenced rather than the individual components. Obviously, bad spelling and grammar can also be used as a combined concept because they do, in empirical fact, often appear together. So # 1 is good grammar.
Nevertheless, I prefer # 3 because I am not sure that bad spelling and grammar is as generally recognized as a single thing as is bacon and eggs.