"How many Finnish children can read?"
Translation:Kuinka moni suomalainen lapsi osaa lukea?
It's because the structure requires you to use moni and it in turn requires the nominative. I admit this kind of structure is tricky because in English it's plural, in Finnish singular.
- kuinka monta lasta considers the children as individual countable objects
- kuinka monta lasta päiväkodissa on? – "How many children are there in the kindergarten" (exact number)
- kuinka moni lapsi then is used when talking partially about the whole
- kuinka moni lapsi osaa lukea? – "How many children can read" (a part of the whole population)
You could see this article on how to use moni, monta, and monet: https://uusikielemme.fi/finnish-vocabulary/interesting-words/moni-monta-monet-many
The "moni" construct simply uses the third person singular. It is akin to the English "many a", as in "many a whale has died this year". Many whales, but singular is used.
In plural it would be "kuinka monet suomalaiset lapset osaavat lukea".
Actually lasta is not a plural form either, even though it is used with many kinds of numerals. Plural in the cases mentioned in this conversation would be lapsia or lapset.
Moni lapsi means something like "many child [nominative form]" and monta lasta is "many child [partitive form]" but both mean the same as "many children." If you want to have "many children" to be also grammatically plural in Finnish, you could say "paljon lapsia" which also means "a lot of children" or "monet lapset," but at least "paljon lapsia" can't be the subject in sentences like this. I'm also unable to explain why, despite being a native Finnish speaker.