Some words in duolingo are nice to know, others just aren´t worth it. Say you´re studying Russian, and you come across the translation of ottoman. That´s not worth learning, really! Words such as these require less effort than others. Important words should be thought about and practiced more. You need to be able to find out which words are the most important, then practice them more. You do this by thinking of which words you would use more often. These are the kinds of words you should work on more than others. For example, if you had to practice either the word teapot or neighborhood, think of which one you would use more. That one´s pretty obvious. But sometimes you would be faced with more difficult choices, like street or park. In cases like this, think of yourself and which word YOU would use more. In the words section, you can practice certain vocabulary. Think of which words are the most important, and be sure to keep your strength bars full on these. I´m not saying that you shouldn´t practice less important words at all, just practice the more important words first.