Questions & Answers about Vain yks kaveri on täällä.
Why is the number written as yks instead of yksi?
This is a very common feature of spoken Finnish (puhekieli). Words that end in -i, especially short numerals like yksi and kaksi, drop their final vowel in everyday speech to become yks and kaks.
I learned that numbers make the following noun partitive. Why is it yks kaveri and not yks kaveria?
You are exactly right that numbers usually trigger the partitive case (like kaks kaveria), but the number one is the exception! The word yksi (or yks) acts like a regular adjective and keeps the noun in the basic nominative case.
Is the word kaveri also part of spoken language?
Yes, kaveri is an everyday, slightly informal word for a friend or a buddy. Because this sentence uses the colloquial yks, pairing it with the casual kaveri sounds much more natural than using the formal word ystävä.
How would I write this exact sentence in formal written Finnish?
To switch back to standard written Finnish (kirjakieli), you simply restore the dropped vowel to make it Vain yksi kaveri on täällä. If you wanted to sound extremely formal, you could also swap kaveri for ystävä.