Hello, dear student!
Continuing the topic on the declension of nouns, it's necessary to mention that there are such nouns in Russian that do not decline in cases. They include the following words:
- ending with -е [e], -и [i], -о [o], -у [u] and stressed vowel -а [a]
Какао [kakao] - cocoa
Желе [zhil'e] - jelly
Такси [taksi] - a taxi
- names of cities, people, borrowed words of feminine gender and the words ending with a hard consonant:
Осло [Osla] - Oslo
Гёте [Gyotekh] - Goethe
Мадам [madam] - Madam
Люксембург [Lyuksimburk] - Luxembourg
- proper names ending with -ова [ova],-ёво [yova], -аго [ago], -яго [yago], -ых [ykh], -их [ikh], -ко [ko]:
Чикаго [Chikaga] - Chicago
Шевченко [Shifchenka] - a Russian surname
There is a category of words in Russian which are modified in plural, for example:
"Человек" [chilavek] and "ребенок" [ribyonak] - a human being, a child
We don't say "человеки" and "ребенки" in plural - it is a mistake. For the word "человек" we use the plural form - "люди" [l'udi] - people, and for the word "ребенок" - "дети" [deti] - children. But you should remember, if there is a numeral before the word "человек", it remains the same form as in nominative case, for example:
Пять человек стояли на улице [P'yat' chilav'ek stayali na ulitse] - Five people were standing on the street.
Восемь человек пострадало при взрыве [Vosim' chilav'ek pastradala pri vzryv'e] - Eight people suffered from the explosion.
As for the word "дети" [deti], it doesn't suit this rule.
Семь/семеро детей [Sem'/semira ditey] - Seven children