I was reading BBC culture page ( See Here ). In the middle of the text, we see:
He became firm friends with the Shakespeare and Company owner in a relationship that lasted until his death 40 years later.
Q1: Does the writer make a mistake?
Q2: Why has the writer used the plural form of "friend"?
To be friends with, to become friends with, or to make friends with are idiomatic phrases. If somebody is your friend, you can say you are friends with him. If you want to start a friendly relationship with a person or persons, you can say that you want to become friends or make friends with him or them accordingly.
So the use of friends in the sentence presented by the writer is correct grammatically.
The writer is correct here.
The plural is being used because there are two people becoming friends with each other: "he" and the owner.
You're right. "I" can't be friends, and "he" can't be friends.
The implication is that "we are friends." Hence the plural for "friends." This is just how the expression is built. You can find it in the Merriam-Webster dictionary.
If you want to use the singular, you'd have to use "of":
I'm a friend of sb.
This is a more distant/less colloquial way of presenting yourself.
To be friends with, to become friends with are idiomatic phrases; you cannot say to be friend with or to become friend with.
If somebody is your friend, you can also say that you are friends with them, for example, "I am friends with Peter"/"She is friends with Sara". Please see the Free Dictionary listing.
"to be friends with someone" seems to be an idiom of AmE. The Free Dictionary quotes the AHD. Oald does not have this idiom.
It may be that "to be friends with" derives from "to be in friendship with".
