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Questions & Answers about Ymmärrätkö sinä hyvin?
Why does the verb ymmärtää (to understand) change to ymmärrä- here?
This is consonant gradation! Ymmärtää is a Type 1 verb with a strong rt in the middle. When you add the -t ending for "you", it closes the syllable. This triggers the weak grade, changing the strong rt to a weak rr. So, the stem ymmärtä- becomes ymmärrä- before adding the -t.
What is the -kö at the end of Ymmärrätkö?
It is the question particle! Adding -ko or -kö to a verb turns it into a yes/no question. We use -kö here instead of -ko because of vowel harmony: the verb has the front vowels y and ä, so the ending must match with ö.
Could I drop the word sinä and just say Ymmärrätkö hyvin?
Yes, absolutely. The -t ending on ymmärrätkö already means "you" (singular), so the pronoun sinä is optional. Including it just adds a little emphasis, like asking "Do you understand well?"
How is hyvin (well) related to the word hyvä (good)?
Hyvin is the adverb form of the adjective hyvä. Just like English changes "good" to "well" when describing an action, Finnish often creates adverbs by adding -in to the base of an adjective.