There are two ways of hiding the words or sentences you already know.
The first method is to:
1. Go to the "Learn" tab and tap on the subtopic that contains the word/ sentence you want to hide.
2. Open the Vocabulary of the subtopic.
3. Tap on the three dots icon. 
4. Tap the switch button near the "Learn This Word" option to hide the word or sentence you don't want to learn. 
The second method is to:
1. Open the Search.
2. Find the word or sentence you want to hide. 
3. Select that word or sentence and tap the switch button near the "Learn This Word" option. 
Note: The hidden words and sentences will not reappear in the learning process. The subtopic containing hidden words or sentences will have a special button in the Vocabulary at the end. If you tap it, the hidden words or sentences will be revealed. 
You can also watch our video tutorial on how to hide the words or sentences you already know here.
How do I unhide the hidden words or sentences?