In the fragments of the text: “they may lose the desire to master whatever task you are asking them to do” (lines 12-13) and “then you have to do more than just present lessons in the app” (lines 18-19), the verb forms in bold express the ideas, respectively, of 
In the sentence of the text: “However, if you want people to continue using the app, and use it frequently and often, then you have to do more than just present lessons in the app” (lines 17-19), the connector However can be replaced, without any change in meaning, by 
The sentence, in Text I, in which the boldfaced expression introduces an idea of addition is 
In Text I, the expression “turn down” in “I couldn’t turn down the great starting salary and a chance to live in New Orleans” (lines 12-14) could be replaced, without change in meaning, by 
