You want to tell your friend to keep a secret. Do you say "Keep it between you and I" or "Keep it between you and me"?
If you have caught yourself asking this question, the answer and explanation for it are quite simple, and the incorrect phrase is a case of hypercorrection.
So, let us not waste any more time. What is the correct phrase? I will tell you, and you do not have to keep it just between you and me.
"So, it's between you and me?"
Yes.
"But why?"
Between is a preposition. It is in the same category of words as to, for, by, with, etc. Prepositions need objects. Here are some examples to illustrate this:
"I talked to him."
"This is for your dad."
"Are you listening to me?"
"It depends on the situation."
"My phone was between the couch cushions."
Let us look at the first example. Would you ever say "I talked to he"? No. Why not? English has subject pronouns (I, you, he, she, we, they) and object pronouns. (me, you, him, her, us, them) Because prepositions require objects, if that object is a pronoun, the object pronoun form must be used.
Here is how we can break this explanation down.
Between is a preposition
Prepositions require objects
If a preposition is followed by a pronoun, the object pronoun form must be used
Therefore, the correct phrase is "between you and me"
So, why do people insist on saying "between you and I"? Probably because it sounds fancy and formal, and must hence be correct! But it is not correct. This is simply a case where we have allowed a hypercorrection to sneak into our brains; we feel that "between you and I" just sounds more proper despite it being completely erroneous.
In the end, it is unlikely that people will ever stop saying "between you and I," and perhaps you will not want to stop them from doing so. However, between you and me, there is only one grammatically correct phrase in this case, and now you know what it is.
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