Learn Finnish: Onko sinulla kahdeksan?

When a number is used all by itself like this, it stays in its basic dictionary form. If you wanted to say "eight of something", the word for that thing would take a special ending (the partitive case, like kahdeksan koiraa for "eight dogs"), but the number kahdeksan would still stay exactly the same.

Why does this sentence use onko ("is") instead of a verb for "have"?

Finnish doesn't have a direct verb for "to have". Instead, it uses the verb olla (to be) plus the ending -lla on the person who owns the thing. Sinulla literally means "on you", so onko sinulla translates literally as "is on you?".

What does the -ko attached to on do?

The -ko suffix is a question particle. When you add it to a word and put that word at the beginning of the sentence, it creates a yes/no question. On means "is" (or in this case, "has"), so onko turns the statement into a question.

Is the word kahdeksan related to kaksi (two)?

Yes! A helpful trick for remembering Finnish numbers is that kahdeksan (eight) historically means "two away from ten". You can see the root for two (kahd-) right at the beginning. Similarly, yhdeksän (nine) starts with the root for one (yhd-) because it is one away from ten.