German mistake: Das habe ich nicht wirklich verstanden”
Correctly said: Das habe ich nicht ganz verstanden.
Influenced by the English language, the problem of the above sentence has nothing to do with spelling or sentence structure, but with the choice of words. “Nicht wirklich” is a literal translation of the English expression “not really”, making the sentence to mean “I did not really understand that”. However “not really” is correctly translated as “eigentlich nicht” in German. Since saying “Das habe ich eigentlich nicht verstanden” would be incorrect, shifting the sentence to mean “I did not quite understand that” works well when translated into literal German, namely - “Das habe ich nicht ganz verstanden.”