What are some examples of nouns that end in -у in prepositional case

Question

I know one already:

in the garden - в саду

Answer 1

in the forest - в лесу
in the nose - в носу
in the heaven - в раю
in the year - в году
in the cupboard - в шкафу
in snow - в снегу
in delirium - в бреду
in common use, widespread - в ходу
in the brain - в мозгу
in ice - во льду
in the mouth - во рту
in the formation - в строю

We study 6 cases at school, and things like this we call exceptions. However, this is locative case from old Russian. Did you notice all these nouns describe where, they point at the location of object?

And a tricky example:

В "Вишнёвом саде" Чехов рассказывает о ...

Locative forms are not used when the noun is a part of a name of a work.

If your understanding of Russian permits, you may read an article about other "hidden" cases that are often considered exceptions.

Answer 2

The ones that come first to my mind (apart from already mentioned) are:

    в aду (in Hell)
    на балу (at the dance party)
    на бегу (while running)
    на берегу (on the shore)
    на боку (on the side)
    в бору (in the pine forest)
    в бою (in the battle)
    на валу (on the bank)
    на возу (on the cart)
    в/на глазу (in/on the eye)
    в году (in the year)
    на [своём] горбу (on [somebody's own] hump, used figuratively = bring something with your own muscles without any help of mechanisms or other people)
    на Дону (in the Don region)
    в жиру (in the fat)
    в зобу (in the craw)
    на кону (at stake)
    в/на краю (in the region/on the edge)
    в кругу (in the circle, mostly used figuratively f.e. в кругу друзей - among friends)
    на лету (while flying, mostly used figuratively = doing something fast and at the same time with something else)
    во/на льду (in/on the ice)
    в мозгу (in the brain, but anatomically speaking we say "в мозге")
    на мосту (on the bridge)
    на мысу (on the cape)
    в/на носу (in/on the nose)
    на плаву (afloat)
    в плену (in captivity)
    в/при полку (in/attached to the regiment)
    в порту (in the port)
    в поту (all over in sweat)
    в раю (in heaven, in paradise)
    во рту (in the mouth)
    на скаку (while riding)
    на слуху (if a person is на слуху, it means he is talked about a lot)
    в/на снегу (in/on the snow)
    в строю (in the formation, in the line-up)
    в/на углу (in/on the corner)
    в хлеву (in the barn)
    в/на ходу (the one which is being commonly used/while moving)
    в/на шкафу (in/on top of the wardrobe)

There are more of them, it's rather hard for me to remember more :D.

Answer 3

I added some more examples of nouns ending with -у in the prepositional (aka locative) case to the Yellow Sky's answer (pending peer's review). There are many more of them still not mentioned.

In this answer I would like to concentrate on the phenomenon of such words, finding common patterns in them, and suggesting the ways of finding them instead of just listing them. Kind of following the Chinese proverb: Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime. :-)

Common pattern

I noticed that for all these words the following is true:

1. An old Russian word
2. Masculine gender, ending with consonant
3. Used in the extended meaning of location inside of something
4. Prepositional case is formed by taking the Dative case of the same word and moving the stress to the ending "-у".

Correct me if I'm wrong but I think in the Old Russian almost all the nouns had such form. With time these forms started to be supplanted by more modern forms of prepositional (locative) case but some words resisted and are still keeping their old forms in certain meanings. This my hypothesis is confirmed by the fact that the words borrowed/created not very long time ago do not have such forms. F.e. compare мёд (old word) and йод (more new). It is possible to say выдержанный в меду, but it will be выдержанный в йоде - not йоду for йод.

The name of the phenomenon

According to Zalizniak's dictionary this phenomenon is called the "2-ой предложный падеж" (second prepositional case). Russian National Corpus website calls it "предложный 2", or, in their English version: "locative 2". (Interesting is that locative - not prepositional - is used as the translation of word предложный for grammatical cases. You can see the "official" list of Russian grammatical cases by clicking "выбрать"/"select" close to the "Грамм. признаки"/"Gramm. features" text box).

All the words that can form locative 2 are marked with the sign П₂ in the Zalizniak's dictionary.

Here is the excerpt from П₂ definition by Zalizniak:

Помета П₂ (за которой может следовать в скобках предлог "в" или "на") - символ
наличия у данного слова особой формы так-называемого 2-го предложного падежа.
...
Помета П₂ означает, что у данного существительного в ед. числе после предлогов 
"в" и "на", выступающих в одном из указанных ниже значений, употребляется особая форма,
отличная от обычного предложного падежа - так называемый 2-ой предложный падеж. Форма 2-го
предложного падежа ед. числа по буквенному составу равна форме Д. ед. того же слова, но 
ударение в ней всегда на окончании. Примеры: "шкаф" (Д. ед. "шкАфу") - П₂ ед. "шкафУ"; 
    ...

See here, and here the original scanned pages.

How to find these words

One possibility is to open the online version of the Zalizniak's dictionary and scan it page by page searching for the mark "п(2)". (For example, look for word "сад" on the page references above). At least this is until a better (for our purposes) version of the online dictionary exists which would allow searching by marks. :-)

Another possibility for finding such words is by using the National Corpus. Click this; click on "select" close to "Gramm. features"; in the following popup check "locative 2". ; on the main page click the "search" button. In the results of search you will see the words in locative 2 case put in the context of their usage.