When dealing with exact scientific measurements in Russian, you will often need to use large compound numbers in oblique cases. The golden rule for declining multi-word numbers is: every single word must decline. If a number has three words, all three words take the case required by the sentence.
The Dative Case and Equivalents
To state parameters, you will frequently use the verb равняться (to equal), which requires the Dative case.
Before looking at a sentence, note that the numbers 40, 90, and 100 are uniquely simple. They only have two forms: the standard Nominative/Accusative form, and an -а ending for all other oblique cases (Genitive, Dative, Instrumental, Prepositional).
| Number | Nominative / Accusative | All Oblique Cases |
|---|
| 40 | сорок | сорока |
| 90 | девяносто | девяноста |
| 100 | сто | ста |
For hundreds (like 800), both halves of the word decline. In the Dative, 800 (восемьсот) becomes восьмистам. Watch how every part of 894 changes in the Dative case below:
The Genitive Case and Extent
When stating that something "reaches" a certain length or capacity, you will often use verbs like достигать (to reach) which take the Genitive case.
For compound numbers in the Genitive case, remember that the "hundreds" words end in -сот (or -ста for 300/400). For example, 200 (двести) becomes двухсот, and 300 (триста) becomes трёхсот.
The Instrumental Case
The Instrumental case has some of the most complex endings. When describing a noun "with" (с) a specific parameter, both halves of the tens and hundreds take Instrumental endings. For example, 600 (шестьсот) becomes шестьюстами, and 80 (восемьдесят) becomes восьмьюдесятью.
The Prepositional Case
When discussing measurements using prepositions like о (about) or в (at/in), apply the Prepositional case to every word. Notice again how 40 (сорок) and 90 (девяносто) simply take their oblique ending -а.