Drank is past simple, drunk is past participle: I drank, I have drunk.
If you ever go through Duo's Spanish tree you'll come across a huge debate started by English-speaking people who think 'I have drank' should be accepted as equally valid because that's how they talk to their friends in their city (it's also prevalent where I come from), but several of them conceded that they wouldn't write it in a letter. I categorise that usage with the custom in my home county of saying 'I aren't': widely used in the region, still not good English.
What kind of exercise was it? Simply translate? Or multiple choice?
Could you next time report the sentence and select that your answer should have been accepted. Or if you are using the app and that isn't an option, take a screenshot and post that here. That way it's easier to see if it's a bug or if there was another issue. Thanks.
Whats the difference between drank and drunk? If you are describing a person who is under the influence, drunk is the word for you. Drank is not used as an adjective. Drank is the simple past tense for the verb drink. Since drank and past are both spelled with the letter A, it should be simple to remember that drank is the simple past tense form of this verb.
"Drank" is not accepted but "drunk" is. Is this an oversight by Duo or a deliberate distinction based on slightly different tenses? http://www.oxforddictionaries.com/definition/english/drank
Drunk is the past participle of "to drink". That is why it's used as an adjective, as in "She is drunk.". Another example is "That is seen as incorrect.", "seen" being a past participle as well.
That's normal for past participles. Also, it's used with a helping verb as in "has drunk" or "have drunk". But, you can't just say "I drunk"....sure, it's kind of colloquial, but I'm American and I'm assuming you are too, but it's still seen as pretty bad everywhere in the U.S. (I've lived and traveled all over the country and know people from every region too.). If you're not American too, ignore the last part.
The simple past is "drank". "I drunk" is incorrect English.