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 appear in the learning process again. The subtopic that will contain 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 again. 