Difficult words are defined by our algorithm, according to how many times you get the word incorrect during testing sessions. After having fully learned and bloomed your word, if you have been marked wrong more times than right, it will be marked as a difficult word.
To consult the 'difficult words' learning mode tap on 'Continue' for a certain course and you will be presented with several different types of learning modes, select 'My Difficult Words'.
What is Memrise Pro?
The Difficult Word Function explained in the website
What can you not do in the iOS App?
Pro features in the iOS App explained