A word or sentence is considered to be learned only when it has participated in all the 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 or sentence, the battery becomes full. When you are beginning to forget a word or sentence, the battery starts to discharge.
What is a word or sentence battery?
How do I make a discharging word or sentence charged again?
How do I know when a topic or subtopic is learned?