Impersonal pronoun (English one/you)

Question

Many languages have an impersonal pronoun ("man" in German, "on" in French). In slightly old-fashioned English, the word "one" is used; as in "Where can one get one's hair cut?" Otherwise, "you" is normally used in English.

I have been taught that in Russian, the common way to express this is to use the second person singular verb without the word ты, as in: "If one knows many people..." = "Если знаешь много людей..."

But I have two problems with this. First, is it proper to use this form with people you would normally address as Вы? Second, how do you handle the word in other cases than the nominative? For example, genitive "get one's hair cut" or accusative "if someone sees you".

Answer 1

"Вы" can be both singular and plural. If you really want plural (can be also impersonative as referring to some "universal" knowledge), you say "если вы знаете" (note the small letter). Yet impersonal construction allows using "ты" as mentioned earlier even if you normally say to the person "Вы". With a capital letter "Если Вы знаете" surely is a personal construction.

Another point that in a spoken language impersonal vs personal "если знаешь" (likewise "если знаете") differs by pronunciation and context only. So it's often not about the choice of words yet how to say them.

Returning to your examples:

"Where can one get one's hair cut?" - Где (здесь) можно постричься? (note that you almost never say "где здесь меня постригут")

"If someone sees you..." - "Если тебя кто-нибудь видит..."

Answer 2

TL;DR:

The less impersonal (generic, abstract) the sentence is, the more you tend to use plural for politeness.

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There is no impersonal pronoun in Russian; impersonality is conveyed with the sentence structure. This is more flexible, and there are three types of such constructs with a common attribute of pronoun missing.

Indefinite-personal is used when the action is carried out by specific person(s), but this is irrelevant. В магазине мне продали водки. (You don't need to mention salespeople did.) Note 3rd personal plural is used for this type of construct.

Generic-personal is more abstract and is used to refer to generalized cases like proverbs. С утра выпил - весь день свободен. (A "wisdom" that suits any alcoholic.) This one goes with 2nd person singular or, occasionally, 3rd person plural.

Impersonal specifies an action or a state without any definite actor. Бутылку водки занесло снегом. (Abstract natural force did this.) With verbs, it is 3rd person singular.

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So, strictly, with the pronoun omitted, the only impersonal/generic construct you can get with 2nd person requires singular, and the use must be abstract as per above.

Colloquially, though, 2nd p. sg. use is not limited to abstract cases. For example, it is very common with directions (идешь квартал прямо, поворачиваешь направо, там парикмахерская). I'd say it's used this way to avoid an imperative that may sound too formal or impolite. Since such use is only marginally impersonal (the directions would work for anyone, but they are given in a particular case), you further go with 2nd p. pl. for politeness.

Finally, there is a colloquial use that further mixes up impersonal and personal by actually getting the pronoun (sg. or even pl.) back into sentence.

Answer 3

In case of "if someone sees you" - you should use "Вы" with people you would normally address this way instead of "ты": "Если Вас увидели" vs. "Если тебя увидели".

But, in case of "one's hair cut" impersonal pronoun is most commonly skipped altogether: "Где бы постричь волосы?" = "Where would one get hair cut?". It's the same in your very good example: "If one knows many people..." = "Если знаешь много людей..."