Translation:My father usually uses the credit card.
Although this is not an English course for Greeks, but Greek for English speakers, I was taught that the proper placement of adverbs in English is at the end of the sentence... "my father uses the credit card usually" should have been accepted...
Your comments and observations are always very helpful and have allowed us to edit or even remove items. Even though we thought we had done a good review we've discovered many things that needed editing, as you will have noticed. So, in this case, you had me rushing off to the grammar books or rather sites and what I see is that while many adverbs come at the end of the sentence adverbs of frequency usually come before the main verb.
I've found several sources and here are two of the most reliable.
The Univ of Michigan: :https://www.press.umich.edu/pdf/9780472032426-unit5.pdf
and the British Council: http://learnenglishteens.britishcouncil.org/grammar-vocabulary/grammar-videos/adverbs-frequency
To cover both Am. Eng and Br. Eng. ;-)
I repeat your input has always been of great value and has helped avoid serious errors. We are all grateful and please keep your comments coming.