Someone (an English teacher) recently told me that I should use one and not you when referring to a class or other group of people. Their (incorrect) reason was that you should only refer to a single person.
One cannot learn English in a day.
You cannot learn English in a day.
This is utter nonsense; both are correct. The word you was not originally singular at all, it was plural. It is also used as a singular word these days. Last week we looked at I and me and the difference between words used as the object and the subject. Among this type of word (pronouns) you is the most flexible; it is used as the subject/ object/ plural/ singular. Both one and you can be used (in the third person) to refer to unnamed people. In this use, one is more formal and much less common and you is used most often.
In modern English, the word you is used for one person (singular) or more than one (plural). It was not always this way. English used to use the plural ye and the singular thou for those doing the action in a sentence (subject). The person receiving the action in a sentence (object) was thee if singular or you if plural. Long ago, when England had been invaded by the Norman French, English was influenced by the French language. In French an important person is referred to in the plural form (vous) and a friend in the singular (tu). (A group of friends still takes the plural form.) Since the English were the ones who were invaded they had to be polite to the French lords or lose their heads. So they referred to a lord or king in the plural, you (or your Lordship, your grace etc., never thy). Even today the English Queen, or other people who symbolise a group, use we instead of I; this use is called the royal we.
To summarise how we can use the word you today:
The person you are talking to:
You have not done your homework. (as singular subject)
They were unkind to you. (as singular object)
A collective group or people in general:
You Mainlanders keep buying all the powdered milk. (as plural subject)
Working in Hong Kong made you expats rich. (as plural object)
In some dialects of English you even replaces your, but don't use that non-standard form.
This history led to thou, ye and thee becoming obsolete in modern English. Thou, ye and thee are still used in some local UK dialects, of which there are many. The old forms are also used when a writer wants to sound formal and traditional, which is odd because thou used to be the informal word. The word you is an example of how English got simplified over the centuries.
by John Larrysson
A native English speaker who has been teaching practical English in Hong Kong for more than a decade.