Answer: conjugation
Explanation: for this you add the verb based on the article. Like for 2, it would be Tu prends le métro, because you are ^taking^ the metro. It gives you the correct verb at the end of the sentence as well, so you just have to modify it so it matches the article. Tu adds an s, On would be kept just as prend or keeping it simple, nous would be prenons, vous prenez, je prends, elle/il prend, and ils/elles prennent. For comprendre it would be, je comprends, tu comprends, il/elle comprend, nous comprenons, vous comprenez, and ils/elles comprennent. It follows the same principles for Apprendre as well :)
