Questions & Answers about Это автобус номер сорок.
Why is the word for forty сорок and not something ending in "-дцать" like 20 or 30?
Unlike 20 (двадцать) or 30 (тридцать), which literally mean "two tens" and "three tens", 40 is a famous exception. Сорок historically referred to a batch of forty animal pelts used as a unit of trade in ancient Russia. You just have to memorize this unique word!
We learned that numbers change the case of the noun. Why isn't there a genitive noun after сорок here?
Because сорок here isn't counting a quantity of anything—it is not "forty buses" (сорок автобусов). Instead, it acts as a label or a name ("bus number forty"). When a number is used as an identifier like this, it doesn't force any words into the genitive case.
Is the word order fixed as автобус номер сорок?
Yes. Just like in English, when you identify something by its number, the word номер and the numeral come after the noun they describe. You wouldn't say номер сорок автобус.
Will I have to learn complicated case endings for сорок later?
Actually, no! Сорок (along with 90 and 100) is one of the easiest numbers to change for cases in Russian. It only has two forms: сорок for the nominative and accusative cases, and сорока for all other cases. Since it is just naming the bus here, it is in the nominative form.