Declension of numerals in Russian. Round numerals

Hello, dear student! Today I would like to analyze the topic on declension of numerals. Once my student has asked me a question at the lesson: "Why do we say "одна бутылка воды" (one bottle of water) but talking about glasses, we use the phrase "два стакана" (two glasses)? To puzzle out these two cases, we should refer to our favorite grammar of the Russian language. We have already known that numerals are the words which denote quantity, numbers or order:

Пятый дом [p'yatyy dom] - The fifth house
Пятеро парней [p'yatera parney] - Five guys
Пять [ p'yat'] - Five

We are going to analyze only round numerals in this article, so let's decline the numeral "сорок" [sorak] (forty) as an example:

Nominative case: сорок [sorak]
Genitive: сорока [saraka]
Dative: сорока [saraka]
Accusative: сорок [sorak]
Instrumental: сорока [saraka]
Prepositional: сорока [saraka]

Such numerals as 40 (сорок - forty), 100 (сто [sto] - one hundred) and 90 [divinosta] - ninety) save their form in Nominative and Accusative; in other cases they change depending on question and end with -a [a].
Compound numerals, such as 50 (пятьдесят [pidis'yat] - fifty), 60 (шестьдесят [shizdis'yat] - sixty, 70 (семьдесят [semdis'yat] - seventy) and 80 (восемьдесят [vosimdis'yat] - eighty) change like nouns in the 3rd declension. Both parts of a numeral change in this case. For example:

Nominative: пятьдесят [pidis'yat] - fifty
Genitive: пятидесяти [pitidisiti]
Dative: пятидесяти [pitidisiti]
Accusative: пятьдесят [pidis'yat]
Instrumental: пятьюдесятью [pit'yudisit'yu]
Prepositional: о пятидесяти [a pitidisiti]

As the example shows, the numeral has changed - we've added the letter -ю [yu] in Instrumental case, in the end and in the middle. The numeral has changed in Genitive, Dative and Prepositional cases in a similar way.

If we want to decline a numeral from 200 (двести [dvesti] - two hundred) to 900 (девятьсот [divitzot] - nine hundred), we should consider that we change both parts of the numeral - the first one like a simple numeral, the second - like a noun in plural. If you forget how to decline nouns, I strongly recommend you begin your study with noun declension and then turn to numerals.
For example:

Nominative: двести [dv'esti] - two hundred
Genitive: двухсот [dvukhsot]
Dative: двумстам [dvumstam]
Accusative: двести [dv'esti]
Instrumental: двумястами [dvum'yastami]
Prepositional: о двухстах [a dvukhstakh]

The example shows, how the numeral 200 (двести - two hundred) has changed - you should memorize this rule. To make the process of study better, I recommend you to practise and decline the following numerals:

400 (четыреста) [chityrista] - four hundred
900 (девятьсот) [divitzot] - nine hundred
70 (семьдесят) [sem'dis'yat] - seventy
100 (сто) [sto] - one hundred