It would not be unusual to hear a grammar teacher (I know one personally) warn students not to rely on their ear in deciding whether a certain construction is correct. Between you and I is a famous example of this. A good number of people would say that phrase is grammatically correct, and a good number would as well (I hope) say no. At the level of grammar, the ear is merely recording what it hears, so we could say that it will be as grammatically correct as are the people it listens to. The ear can alert, certainly, but it cannot judge.
This injunction against the grammatical ear is ironic: it’s not that the ear as a faculty of perception is inferior; it’s actually too sophisticated an instrument to be deciding elemental points of grammar and logic. Things can sound right and be wrong, whether the standards of right and wrong are icily objective as in logic and math, or whether they are right for a time as in grammar and style. Take, for example, the notoriously confusing noun people. I heard someone say the other day, but people doesn’t believe that. His ear approved of that construction, and to compound the confusion, he was knowledgeable enough in English grammar to justify the clause: the noun people is singular in form and therefore requires the singular verb doesn’t. Good thought, wrong answer.
We know that nouns in English and elsewhere (though not everywhere) have a property called number. They are either singular when they refer to one thing, or plural when they refer to more than one. To mark the difference, we usually add the letter s to the singular, or we change the spelling of the singular in some other way. So, for example, the noun person is singular, and we form its plural simply by adding the letter s: persons. Our ear says, yes, of course, and we move merrily on. Until we come to the noun people. We could see that noun as singular (no letter s has been added), and so decide confidently to combine it with a singular verb to meet the regular grammatical obligation that subject and verb agree in number. My ear might start sending up the yellow flags, your ear, like the person I was speaking with, might not. So what to do? Acknowledge the alert and understand the grammar.
The singular form of the noun people can serve as the plural of the noun person. We can speak equally of two persons in the kitchen or two people in the kitchen. This was not always so, but the standards of language are often a moving target (which is one of the reasons we keep reading: to see what better writers are doing). Here, the word people, meaning human beings, is singular in form but plural in meaning. And if that weren’t enough, the same word people in the same singular form is also used to mean a group of persons who share some cultural trait, for example, a language or set of customs. We can speak, for example, of the Americans, an English-speaking people. Here, the word people is a collective noun, singular in form but referring to a collection of similar units; and if we pluralize the collective noun people by adding an s, we multiply the collections: the continent includes many distinct peoples.
In matters of grammar, the ear serves best when it suggests we pause to consider what we’ve heard or read. Then we apply our skills of analysis and come to a reasoned decision. So where does the ear come into its own? In the subtle matters of rhetoric, where the standard is judged according to the effect we intend to have on a certain audience, not on a body of fairly consistent rules. Certainly the two spheres of grammar and rhetoric overlap, but it can help us tremendously to employ the right tool for the right job: our eyes and mind for the rational work of grammar; our ears and heart for the impression we wish to leave.