Difficult words are defined by our algorithm, according to how many times you get the word incorrect during testing sessions. After having fully learned your word, if you have been marked wrong more times than you were marked right, this particular word will be detected as difficult, so you can practise it further in this special session to make sure it sticks in your mind.
Please note: Difficult Words is a Pro mode - to find out more about Memrise Pro please read these articles!
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