Tap on the sentences you want to highlight to block them. You can't underline. If you want to highlight more than one sentence such a whole paragraph, you must tap each sentence in the paragraph. Once you have them all tapped (they will be in blue, at least on my devices) look to the lower right corner. You will see what looks like a 4 bars stacked. Tap on that which brings up a menu. Choose what you want to do with the sentence(s) you blocked, and tap your choice. You can use a highlighter and pick the color you want. You write a note, or bookmark it. It's pretty simple. If you want to say, highlight and make a note, just block the same sentence(s) a second time and choose again. If you tap the little house symbol in the upper left, that will take you to the menu. Tap on the study center to see all your bookmarks, highlights, notes, etc. You can also delete them if you want.