Declining Higher Numbers
When counting inventory or office supplies, you will often need to use numbers in cases other than the Nominative.
Numbers from 5 to 20 decline exactly like feminine nouns ending in -ь (the 3rd declension). For the number 50 (пятьдесят), you must decline both halves of the word. The number 100 (сто) is the easiest—it only has two forms!
Here is a quick reference for how these numbers change:
| Case | 5 to 20 (e.g., 5) | 50 | 100 |
|---|
| Nom / Acc | пять | пятьдесят | сто |
| Gen / Dat / Prep | пяти | пятидесяти | ста |
| Instrumental | пятью | пятьюдесятью | ста |
Genitive, Dative, and Prepositional Cases
In the Genitive, Dative, and Prepositional cases, the -ь at the end of numbers 5 through 20 changes to -и. Because 50 is made of two parts, both parts get the -и ending: пятидесяти.
A very common time to use the Genitive case with numbers is after the preposition около (about / approximately).
| approximately, about | около |
The Number 100 (сто)
As shown in the table, the number 100 is remarkably simple. Aside from its Nominative and Accusative form (сто), it becomes ста in every other case.
The Instrumental Case
When using these numbers to say you are working with a certain amount, you will need the preposition с + the Instrumental case.
For numbers 5 to 20, the ending becomes -ью (e.g., двадцатью). For 50, both halves take the ending: пятьюдесятью. The number 100 remains ста.