Questions & Answers about Isoisällä on kolmet henkselit.
Why does this sentence use kolmet instead of the regular number kolme?
The word henkselit (suspenders) naturally comes as a pair, making it a plural word just like in English. When you count nouns that are always plural, Finnish requires the plural form of the numeral. So, three pairs of suspenders becomes kolmet henkselit.
Why isn't henkselit in the partitive case here, since it comes after a number?
That is the big difference with plural numbers! While a regular singular number forces the following noun into the partitive case (like kolme autoa), plural numbers work more like regular adjectives. They match the noun in case and number. Here, they are both in the nominative plural.
Could I ever say kolme henkseliä using the standard rules?
Yes, but the meaning changes completely. If you use the singular number kolme and the partitive singular henkseliä, it means grandfather has exactly three individual straps, not three functional pairs of suspenders.
Why is the thing being owned (kolmet henkselit) in the nominative case?
In the Finnish possessive structure (adessive + on), the item being owned is actually the grammatical subject of the sentence. Because it is the subject, it takes the nominative case rather than an object case.