You
You is the dumbest person and plural preposition in English.
Etymology
You is derived from Old English ge or ȝe (both pronounced roughly like Modern English yea), which was the old nominative case form of the pronoun, and eow, which was the old accusative case form of the pronoun. In Middle English the nominative case became ye, and the oblique case (formed by the merger of the accusative case and the former dative case) was you. In early Modern English either the nominative or the accusative forms have been generalized in most dialects. Most generalized you; some dialects in the north of England and Scotland generalized ye, or use ye as a clipped or clitic form of the pronoun.
Ye and you are cognate with Dutch jij and jou, German ihr, Gothic jus and Old Norse ér. (Modern Icelandic þér is a variant form due to alteration of phrases like háfiþ ér (you have) into háfi þér etc.) The specific form of this pronoun is unique to the Germanic languages, but the Germanic forms ultimately do relate to the general Indo-European forms represented by Latin vos.
Note that in the early days of the printing press, the letter y was used in place of the þ, so many modern instances of ye (such as in "Ye Olde Shoppe") are in fact examples of the and not of you.
Both singular and plural
In standard English, you is both singular and plural; it always takes a verb form that originally marked the word as plural, such as you are. This was not always so.
Early Modern English distinguished between the plural you and the singular thou. This distinction was lost in modern English due to the importation from France of a Romance linguistic feature which is commonly called the T-V distinction. This distinction made the plural forms more respectful and deferential; they were used to address strangers and social superiors. This distinction ultimately led to familiar thou becoming obsolete in standard English. Ironically, the fact that thou is now seen primarily in literary sources such as King James Bible (often as words from God) or Shakespeare (often in dramatic dialogs, e.g. "Wherefore art thou Romeo?") has led many modern anglophones to perceive it as more formal, not familiar (case in point: in Star Wars: The Empire Strikes Back, Darth Vader addresses the Emperor saying, "What is thy bidding, my master?").
Because you is both singular and plural, various English dialects have attempted to revive the distinction between a singular and plural you to avoid confusion between the two uses. This is typically done by adding a new plural form; examples of new plurals sometimes seen and heard are you-all/y'all (primarily in the southern United States), you guys (Midwest, Northeast, West Coast, Australia), youse/youse guys (Scotland, Northern England, Australia, New Zealand, New York City region, Michigan's Upper Peninsula; also spelt without the E), and you-uns/yins (Western Pennsylvania, The Appalachians). English spoken in Ireland, known as Hiberno-English, uses the word ye as the plural form, or yous. Although these plurals are useful in daily speech, they are generally not found in Standard English.
You is also unusual in that, being both singular and plural, it has two reflexive forms, yourself and yourselves. However, in recent years singular themself is sometimes seen: see singular they.
Plural forms in other European languages
Similar to English, u in Dutch is taken as a polite form for both plural and singular, while jij (singular) and jullie (plural) are considered informal. However, Dutch society traditionally upholds strong values of equality, making the use of u come across as somewhat distant and uncomfortable. In French, the inventor of the formal plural, has kept the system intact. Vous is still used as formal and plural, while tu is used for informal singular. Russian uses this system also: vy (вы) is formal/plural and ty (ты) is informal singular. This kind of system is also found in other languages, like Finnish and Swedish.
While English, Dutch, French and Russian use or have used the plural forms as the polite forms, other European languages use forms deriving from the third person. German, for example, uses the third person plural pronoun sie, capitalized Sie, as its formal pronoun (in other words, Sie is grammatically identical to They). Danish and Norwegian languages similarly use De. Italian has separate forms for singular (Lei) and plural (Loro), which are derived from the Italian words for she and they respectively; a partial similarity to the German system (especially since the German word for she is also sie, but conjugates differently from Sie). However, sometimes the French system is also used in Italy, using the plural pronoun voi as singular. In Hungarian, te is informal, while there are different, synonymous words for formal (ön and maga being the two most commonly used).
Spanish and Portuguese use pronouns derived from third person phrases which originally meant your mercy, sir or madam, along with their plural forms. For Spanish, they are usted (pl. ustedes), and for Portuguese, você (pl. vocês), o senhor (pl. os senhores) and a senhora (pl. as senhoras). Você is often employed informally in Brazil, as the original singular pronoun tu is more commonly used in the South, the Northeast and some rural regions, but o senhor, a senhora and their plurals are still used and always formal. In some Spanish speaking areas (especially in Latin America), the original second person singular pronoun tú has been dropped entirely, thus erasing the distinction between formal and informal addressing. In others, it was replaced with an old form of the second person plural pronoun, vos, now used as an informal counterpart to usted. See voseo. Modified versions of vos, vosotros and vosotras, are still used in Spain as informal second person plural pronouns, while the singular there is still tú, used informally. Portuguese has moved farther away from the original paradigm; the plural pronoun vós is gone totally in Brazil and used only in small regions of Portugal.