Questions & Answers about Oletko sinä rakentanut pöydän itse?
How is the past participle rakentanut formed here?
Rakentanut comes from the Verb Type 1 verb rakentaa (to build). To form the active past participle for Type 1 verbs, you take the infinitive stem (drop the final -a or -ä) and add -nut or -nyt. Here, rakenta- becomes rakentanut.
Why does this sentence use oletko ... rakentanut instead of the regular past tense?
Oletko rakentanut translates to the perfect tense "have you built". Just like in English, Finnish uses the perfect tense to talk about past events that are still relevant to the present moment. It is formed using the helper verb olla (here, oletko) plus the past participle rakentanut.
Why is "table" written as pöydän instead of its basic form pöytä?
Because the table is the object of the sentence and the building action is complete (you built a whole table), it takes the genitive-like accusative -n ending. Adding this ending closes the syllable, which triggers consonant gradation and softens the strong t in pöytä to a weak d in pöydän.
Why use the pronoun sinä when oletko already means "have you"?