Quote Originally Posted by George1992 View Post
It surprised me that you put an adverb behind "довольно" because In Czech it is possible to say "Я довольно знаю" in this very context.
«Довольно знаю» is possible in Russian too, but sounds a bit old-fashioned.«довольно знаю» in RusCorpora. Notice the years.

In modern language, the adverb довольно is used mostly with adjectives and other adverbs.

Quote Originally Posted by George1992 View Post
I would also like to ask you about the sentence "Спроси у моей сестры". Is it necessary to say "у"?
You can say «спроси у моей сестры» («у» + genitive) or «спроси мою сестру» (accusative).