There are two ways to unhide the words or sentences you have hidden.
The first method is to:
1. Go to the "Learn" tab and select the subtopic that contains the hidden word/ sentence.
2. Tap on the Vocabulary icon.
3. Reach the end of the Vocabulary of that subtopic. 
4. Tap on the "+1 hidden" (NOTE: The number may differ).
5. Tap on the three dots icon. 
6. Tap the switch button near the "Learn This Word" option.
The second method is to:
1. Open the Search.
2. Find the word or sentence you have hidden.
3. Select that word or sentence and tap the switch button near the "Learn This Word" option.
How do I hide the words or sentences I already know?