Nouns denote things, and they carry the information in a text.
Verbs denote activities, things that happen or what someone does.
Adjectives describe what things look like. They are linked to nouns and modify their meaning in describing or limiting them.
We mainly use these three parts of speech when we pick keywords, whatever note-taking technique we use, simply because they provide us with the information we need. The nouns are the most important ones. This does not mean you should exclude other words. Keywords are simply the words that give you the best conclusions .