It is useful to annotate a text by highlighting key information, underlining new words and then defining them, and circling important numbers or statistics. Anohter way to annotate text is to note down the main idea of each paragraph in the margins,

and then add any key points under these main ideas. This is especially useful when studying for an exam, as you can easily see the important details you noted down when you reread the passage.

这是国外原版摘录的，用以解释annotating text

上面文中出现了annotate a text, annotate text, 怎么理解text, a text？还有一点，noted down 用了一般过去时怎么回事？改用have noted down现在完成时可以吗？