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Questions & Answers about Onko vesi puhdistunut?
Where does the verb puhdistua come from?
It is formed using the automative suffix -tua. The base word is the adjective puhdas (clean). From that, you get the active verb puhdistaa (to clean something). By swapping the ending for the -tua suffix, it becomes puhdistua (to become clean), meaning the subject undergoes the change on its own.
Could I use puhdistaa instead of puhdistua in this sentence?
No, because puhdistaa is a transitive verb, meaning it needs a direct object (someone has to clean something). Since the water isn't cleaning something else, but rather becoming clean itself without a specific person acting on it, you must use the automative verb puhdistua.
Why does the main verb end in -nut (puhdistunut)?
This sentence is in the perfect tense, which translates to "has become". The perfect tense is formed with the auxiliary verb olla (here as the question onko) plus the past participle of the main verb. For a verb like puhdistua, you form the active past participle by taking the infinitive stem and adding -nut, giving you puhdistunut.
Why is the subject (vesi) placed between onko and puhdistunut?