How do I "master" something?

Clozemaster uses a spaced-repetition system to help you remember what you learn more efficiently. Every sentence starts at 0% Mastered. When answered correctly, it moves to 25% Mastered and is queued up to be reviewed again in the future. The default review intervals are

So in order to get a sentence to 100% Mastered you need to answer it correctly 4 times in a row over an increasing period of time. With Clozemaster Pro users you can customize these intervals to whatever number of days best suits your learning style.