In English, we have one word for both the second-person singular and second-person plural: "you." That can lead to ambiguity in some cases, so some English speakers adopt informal work-arounds -- "you guys," "y'all," and other phrases have entered our lexicon.  (I tend to use "you all" for plural, for what it's worth.) But many other languages use different words for the singular and plural cases. Additionally, English speakers, when using second-person pronounces, do not have a way to signal an informal relationship (e.g., close friends and family) and more formal ones. Compare English to Spanish, and you'll immediately see the differences. In Spanish, you'd say "tú" when speaking to a close friend and "vosotros" when talking to a group of friends, but for, say, your teachers, you would say "usted" for one instructor and "ustedes" for a bunch of them. Four words, all meaning "you." And which one you use matters, and not just for precision's sake. Using "tú" when you should have used "usted" can be downright insulting.

But if you think getting rid of "tú" to avoid that problem is a good idea, well, think again. Just ask Sweden. 

The Swedish language has two words for "you," one for the second-person singular and another for the second-person plural. The singular version is "du" and the plural is "ni," but before 1967, "du" was only used in informal situations -- as Slate reported, "'du' was acceptable with children only, and of course with people for whom one had no respect." If you were speaking to a group of people, you could use "ni" for both informal and formal purposes, but if you were speaking to someone specific, you couldn't use "du" without insulting them. As a result, Sweden had a gap in its language; effectively, there was no singular second-person pronoun available for informal use.

The language filled the gap by using third-person proper nouns -- titles, last names, and even professions -- as second-person pronouns. For example, if you wanted to know if a senior executive at your company wanted a cup of coffee, you'd likely say the equivalent of "would Mr. Cheif Executive Smith like a cup of coffee," with "Mr. Chief Executive Smith" filling in for the word "you."  And if you didn't know the person's last name or profession, you'd kind of talk around the topic, saying something like "Might a cup of coffee be liked?". It was clunky, to say the least.

But it was, culturally, all but required. That was, until senior leaders decided to lead by example and ask people to refer to them as "you." The initial efforts were centered around expanding "ni" such that it could be used in the singular as well as the plural; as it had already been an acceptable way to refer to someone formally, many figured that using "ni" in a formal, singular sense would work. But it didn't -- using it to apply to only one person often offended the person being spoken to. (It's unlikely that the people saying "ni" were doing so to be mean, and it's similarly unlikely that they would demand a sacrifice of a shrubbery to appease them if they did.) So in 1967, they gave it another try. Bror Rexed, a neuroscientist, was appointed to head Sweden's National Board of Health and Welfare that year, and in his onboarding address, asked the staff to refer to him using the word "du." This flipped the stigma -- previously, using "du" when speaking to him would have been an insult; now, not using "du" would be insubordinate. 

The Swedish population embraced the change. As Slate explains, "When the revolution came, it came fast. In the early ’60s, prudence still reigned. But by the close of the decade, even the prime minister had been du’d, like anyone off the street. Only the royal family remained out of range." And the change stuck -- today, you'll rarely find anyone in Sweden hesitate to use "du."