English is common and useful because it is common. However like other languages it has not been well planned or planned at all. It is the product of history and accident. One problem is that the set of pronouns are flawed. They cannot be used clearly for every situation. Pronouns are those words used to replace a name, or names, to make speaking faster and easier; for example: me, you, her, their....
The pronoun you is used for both one person or more than one person that is being talked to. Some English dialects have created new words for a plural of you: y'all, yinz, you-uns, you guys, you lot, yous, youse.... None of these is formal English and they are not often accepted (or understood).
A clever student said, “English does not make sense!”
I answered, "Yes, but why?”
He said, “When the subject is one person, such as a person's name, he, she or it we use is. That is easy enough. Then if the subject is many people, such as they or we, we use are.”
I asked, “Yes, what is the problem?”
He answered, “The word you is a problem. It is one person, but we use are, not is.”
I explained, “The word you is more than one person.”
He was surprised. “What? I am talking to you and you are one person.”
So I explained, “I can use the word you to mean the whole class. For example, 'You have an English test on writing dialogue next week'. We used to use the word thou to mean one person. But we now use the (plural) word you for one person to be polite because of an historic problem*. Most people have forgotten that there was another word for you.”
He asked, “Can we still use thou for one person, instead of the word you?”
I answered, “Yes you can. It is not wrong, but it does sound very old-fashioned.”
* The Norman Conquest (Google: John Larrysson Norman site:english.mingpao.com)