A word or sentence is considered to be learned only when this word or sentence has participated in all the existent games from the Learn Section (Words- 6, Sentences- 10) and you have provided correct answers in all of them.
The battery of each word and sentence is starting to charge with every correct answer.
When you have learned a word, the battery becomes full.
When you are starting to forget a word, the battery is starting to discharge.
What is a word/ sentence battery?
How do I make a discharging word or sentence charged again?
How do I know when a Topic or Subtopic is learned?