The Russian word for soup is суп. It is an end-stressed noun in the plural, but stem-stressed in the singular, which means it declines like this:

The Russians love soup, and frankly the best soups I've ever had in my life have been in Russia. That's because the Russians aren't worried about things like heart disease and cardiac plaques: there's plenty of fat or butter in every soup. And soup is an essential part of обед. If you try to serve a Russian обед and there's no soup involved, you can be darn sure they think you are stinting on your hospitality. The most famous Russian soups are a beet soup called борщ (see Olga's entry about it) and a cabbage soup called щи.

Although for classroom purposes we say that Russian has six cases, in fact it has quite a few more. Some nouns distinguish a locative case that is separate from the prepositional case, and суп is one of them. That is, sometimes instead of «в супе» you may encounter «в супу» with the meaning “in the soup.” The locative case is slowly dying out in Russian, merging with the prepositional. «В супе» is much more common these days than «в супу».

You may also encounter a partitive case form супу, which means “some soup” as in «Хочешь супу?» “Do you want some soup?” or «На завтра хозяин сварил грибного супу из шампиньонов» “For tomorrow the host has made some mushroom soup from champignons.” The partitive case is also dying in Russian, merging with the genitive case. Mostly you will find the plain genitive case in such contexts nowadays.

Sample sentences:

¹ Кто in Russian applies to animals as well as people, so the sentence means “What kind of critter is floating in my soup?”