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The "Currently Reading" tab gives users the option of fetching new sentences for the current study focus vocab (highlighted in blue both in the study focus category and in the current sentences). The yellow highlight greatly increases the locus of attention to certain warnings or instructions under the "fetch next sentence" button. Note that each sentence block is separated by alternating blue/white colors and the "Close" button for each sentence block. Clicking on the "Close" button will move the corresponding sentence block to the "Closed Sentences" tab, in a stack-like fashion (chose for better feedback). 
The decision was made to allow all words in each sentence to have the affordance of a button and be clickable to display vocab information and manipulation options. The goal was to increase both the efficiency and learnability of the design, allowing users to find out information about vocab words with a simple click (learnability) and allowing users to efficiently make a vocab the study focus or "allow word to be displayed in sentences" (efficiency) without having to search for the word on the left hand side.
The "Closed Sentences" tab serves almost like a recycling bin. Creating a place for users to dispose and restore sentence blocks. Additionally, to improve efficiency in terms of scalability, a search option is added to allow users to filter through closed sentences, by either PinYin, English, or Chinese. The reason a "Closed Sentences" tab was implemented, as explained above, is due to overwhelming reader request for efficiency and navigation. Although this increases the complexity of the design, it offers better visibility and user control since it allows the user to "clean up" the "Currently Reading" list of sentences.
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