"I ask only once."
Translation:Eu peço somente uma vez.
Both peço (pedir) and pregunto (preguntar) should be acceptable here, since there is no context given. The most common context in English would be when you say to your child, "I am only going to ask you once to stop doing that!"
Perguntar = to ask a question
Pedir = to ask for something (such as a favor)
Eu somente pregunto uma vez is as right as Eu pregunto somente uma vez, why does it take it as wrong?
Oh yes.... Spanish may confuse our heads when learning Portuguese... :S