Questions & Answers about Minä ostan usein viiniä.
Why is the word for wine viiniä here instead of just viini?
It is in the partitive case! You use the partitive for uncountable things (mass nouns) like food and liquids when you are dealing with an unspecified amount. Since you are buying "some wine" rather than a specific whole bottle, you add the partitive ending -ä to the noun viini.
Why does viiniä end in -ä rather than -a?
This is due to vowel harmony. The word viini only contains the vowel 'i'. The vowels 'i' and 'e' are neutral, and when a word only has neutral vowels, Finnish rules say it must take the front vowel versions of endings. Therefore, it takes -ä instead of -a.
In English we say "I often buy", but here it is ostan usein ("buy often"). Is this word order mandatory?
Generally, yes. In Finnish, adverbs of time and frequency like usein (often) usually come immediately after the verb they describe. Saying Minä usein ostan sounds unnatural to a Finnish speaker.
What is the dictionary form of the verb for "to buy", and how did we get ostan?
The dictionary form is ostaa. To make the "I" form, you find the verb stem by dropping the last letter, giving you osta-. Then, you add the personal ending for "I", which is -n, resulting in ostan.